Trigonometric functions?

Okay, since I haven't studied to much in depth about trigonometry and trigonometric functions, can someone explain to me the functions of sin, cos, tan, cot, csc, sec and their applications? I usually always ask myself, "Where did people derive this from?" and "what will using these functions help me find out about a triangle?". Sure, I could just follow what it says and do my homework and be over with it, but I like to fully understand and grasp concepts such as this.