OK, watch this:
Suppose I have a weight on the end of a spring. Assuming the spring obeys Hooke's law, as the weight is displaced from its rest position, the spring exerts a restoring force in the opposite direction who's magnitude is equal to the displacement multiplied by the spring constant.
Suppose that $f(t)$ represents the displacement of the weight at time $t$. If we assume that the spring constant and the mass of the weight are both unity, we have
$$f''(t) = -f(t)$$
This is an equation involving both $f$ itself and its derivative $f''$, so this is presumably a "differential equation". I don't know how to deal with such a thing. But it is clear that this does not yet tell me what $f$ is, only that it must satisfy a specific property.
Thinking about this for a moment, it is clear that $f(x) = 0$ has the requested property. This corresponds to the weight remaining stationary for all eternity - a physically valid, but rather "boring" result.
Contemplating this further, it occurs to me that the derivative of $\sin$ is $\cos$, and the derivative of $\cos$ is $-\sin$. So if $f(t)=\sin(t)$ then $f''(t)=-\sin(t)$, which satisfies the equation. By nearly identical reasoning, $f(t)=\cos(t)$ would also work. In short, if you ping the weight, it oscillates around zero.
Now suppose that, by some bizarre mechanism, the restoring force were to somehow be in the same direction as the displacement. Impossible, I know. But imagine. Now our equation becomes
Again $f(t)=0$ would work. But what else? Well, there is exactly one function who's derivative equals itself: $\exp$. This is a stronger property than we need, but still, if $f(t)=\exp(t)$ then every derivative of $f$ (including $f''$) would equal $f$. This describes the weight accelerating away exponentially - rather as you might expect.
So far, we have two equations. The solution to one is $\sin$. The solution to the other is $\exp$. Two similar equations, two totally different solutions. Or at least, they look different. But now I'm thinking about something Euler once wrote:
$$\exp(ix) = \cos(x) + i \sin(x)$$
Say that, and suddenly these solutions don't look so dissimilar at all. Now they suddenly look suspiciously similar!
My question: Is this the result of some deep and meaningful connection? Or is it merely a coincidence?
Holy smokes, you guys are right!
I know, of course, of $\sinh$ and $\cosh$. (For real numbers, they look utterly unrelated. But in the complex plane, one is a trivially rotated version of the other.) What I didn't know, until I looked it up, was the derivatives of these functions.
Since they're defined in terms of $\exp$, I was expecting some really complicated derivative. However, what I actually found (as you presumably all know) is that $\sinh'=\cosh$ and, unlike in the circular case, $\cosh'=\sinh$!
So yes, for $f''=-f$ we have $f=\sin$ or $f=\cos$, and for $f''=f$ we have $f=\sinh$ or $f=\cosh$. So flipping the sign of the differential equation rotates the function in the complex plane. Physically, it doesn't look very meaningful to talk about complex-valued seconds, but mathematically it all looks vastly too perfect to be mere coincidence.