All of the functions you have asked about are called Trigonometric functions. They are all based on relationships between angles and sides of triangles. As such they are related. For example $\tan(x)=\sin(x)/\cos(x)$. These functions don't provide a measure of an angle, as you can see from the $\tan(x)$ example. However, there inverse functions do.
However, overtime, they were found very useful in many of the sciences we use today such as Astronomy, Electronics, Mechanics, Wave Theory, Telecom, Navigation Systems,etc.
You can find good detail in this link: Wiki-Trig
A somewhat related question came about lately in here: What is so special about Sin(x)