How would you describe calculus in simple terms? I keep hearing about this weird type of math called calculus. I only have experience with geometry and algebra. Can you try to explain what it is to me?
 A: One of the greatest achievements of human civilization is Newton's laws of motions.  The first law says that unless a force is acting then the velocity (not the position!) of objects stay constant, while the second law says that forces act by causing an acceleration (though heavy objects require more force to accellerate).
However to make sense of those laws and to apply them to real life you need to understand how to move between the following three notions:


*

*Position

*Velocity (that is the rate of change in position)

*Acceleration (that is the rate of change of the velocity)


Moving down that list is called "taking the derivative" while moving up that list is called "taking the integral."  Calculus is the study of derivatives and integerals.
In particular, if you want to figure out how objects move under some force you need to be able to integrate twice.  This requires understanding a lot of calculus!
In a first semester class you usually learn about derivatives and integrals of functions of one variable, that is what you need to understand physics in 1-dimension!  To understand the actual physics of the world you need to understand derivatives and integrals in 3-dimensions which requires several courses.
A: Calculus is basically a way of calculating rates of changes (similar to slopes, but called derivatives in calculus), and areas, volumes, and surface areas (for starters).
It's easy to calculate these kinds of things with algebra and geometry if the shapes you're interested in are simple.  For example, if you have a straight line you can calculate the slope easily.  But if you want to know the slope at an arbitrary point (any random point) on the graph of some function like x-squared or some other polynomial, then you would need to use calculus.  In this case, calculus gives you a way of "zooming in" on the point you're interested in to find the slope EXACTLY at that point.  This is called a derivative.
If you have a cube or a sphere, you can calculate the volume and surface area easily.  If you have an odd shape, you need to use calculus.  You use calculus to make an infinite number of really small slices of the object you're interested in, determine the sizes of the slices, and then add all those sizes up.  This process is called integration.  It turns out that integration is the reverse of derivation (finding a derivative).
In summary, calculus is a tool that lets you do calculations with complicated curves, shapes, etc., that you would normally not be able to do with just algebra and geometry.
A: Calculus is a field which deals with two seemingly unrelated things.
(1) the area beneath a graph and the x-axis.
(2) the slope (or gradient) of a curve at different points.
Part (1) is also called 'integration' and 'anti-differentiation', and part (2) is called 'differentiation'.
A: There came a time in mathematics when people encountered situations where they had to deal with really, really, really small things.
Not just small like 0.01; but small as in infinitesimally small.  Think of "the smallest positive number that is still greater than zero" and you'll realize what sort of problems mathematicians began encountering.
Soon, this problem became more than just theoretical or abstract.  It became very, very real.
For example, velocity.  We know that average velocity is the change in position per change in time (i.e., 5 miles per hour).  But what about velocity at a point in time?  What does it mean to be going 5 mph at this moment?
One solution someone came up with was to say "it's the change in position divided by the change in time, where the change in time is an infinitesimally small amount of time".  But how would you handle/calculate that?
Another problem came about trying to find the area under a curve.  The current accepted solution was to divide the curve into rectangles, and add together the area of the rectangles.  However, in order to find the exact area under the curve, you'd need to divide it into rectangles that were infinitesimally tiny, and, therefore, add up an infinite amount of tiny rectangles -- to something that was finite (area).
Calculus came about as the system of math dedicated to studying these infinitesimally small changes.  In fact, I do believe some people describe calculus as "the study of continuous changes".
A: To be very brief and succinct:

Calculus is the study of how quantities change

Slightly more technically, it a subject based on infinitesimals.
It may be pointing out the obvious, but the Wikipedia article does actually provide a pretty decent beginners introduction to the subject. You'll generally want to start with differential calculus and move on quickly to integral calculus, followed by linking up the two (fundamental theorem of calculus) and moving on from there.
A: Calculus is the mathematics of change. In algebra, almost nothing ever changes. Here's a comparison of some algebra vs. calc problems:
algebra: car A is driving at 50 kph. How far has it gone after 6 hours?
calc: car B starts at 10 mph and begins accelerating at the rate of 10 kph^2
(kilometers per hour per hour). How far has car B gone after 6 hours?

Note how the algebra problem nothing changes, where in the calc problem, the speed of the car is constantly changing.
calc: If a ball is rolling in a straight line at 10 fps with a diameter of 1 foot
and Q is a the point at the top of the ball when t=0, how fast is point Q moving 
at time t=4 relative to the ground?

The speed of the point in relation to the ground is never the same (its zero when its at the bottom, 20fps when it's at the top. Calculus lets you figure out how fast it's going exactly at a specific moment. 
There are two main branches of calculus, differential and integral. These problems pertain to differential calculus as they concern how something is changing. Integral calculus deals with how much something has changed, the opposite of differential calculus. 
The number of cubic feet of oxygen circulated by Jeff lungs per hour at time t 
follows the equation f(t) = t^3 + sin(t) How many cubic feet of oxygen do 
Jeff's lungs cycle per day?

To find out how much something has changed when its rate of change isn't constant requires integral calculus. (the equation is purely hypothetical unless Jeff happens to be the size of a beluga whale).
A: Have a look at this explanation
http://betterexplained.com/articles/a-gentle-introduction-to-learning-calculus/
I hope you love it :)
A: r/eli5 moots the same question, and I'll quote my favorite answer by tud_the_tugboat:
:

I see a lot of people here talking about finding slopes and rates, and all of this is correct. There's also people mentioning the area or space under a curve/surface, which is also calculus.
All of this is true, but I want to add something that gets at the beauty of calculus a bit more, and doesn't even require notion of functions!
At its heart, calculus is the relationship between change (ie. rates, slopes, differentials) and content (ie. volume, area, distance, etc). It's a field that connects how big something is to how much it grows when small changes are made or, conversely, how knowing the rate that something is changing can tell you how much "stuff" you've accumulated.
For example, pretend you're in a vehicle where you can't see out the window. The only thing you can see in the car is the speedometer. As the car drives, you can keep track of the speedometer at every point in time and you'll know how much distance the car has traveled without being able to measure the distance of the car's path.
I think it's beautiful that calculus connects two seemingly unrelated: change and content. This is what math is in general though - it is the study of taking seemingly disparate things in the world and showing that they are fundamentally connected.
A: This is the simplest explanation. 
http://www.math-prof.com/Calculus_1/Calc_Ch_01.asp
ie...its simply a way of dealing with an equation where the denominator has become 0. Instead of leaving it as insoluble...you have a method of proceeding.
