sin, cos and tan are used in trigonometry and to calculate angles in right angle triangles
in a right angle triangle there is the hypotenuse, opposite and adjacent
sin= O/H
cos=A/H
tan=O/A
the slash being the dividing thingy, dont know wat its called :P
there are also certain formulas where u can work out angles, and length of sides, etc such as the sine rule and the cos rule
if you want a more detailed answer its best if you look it up in a maths textbook because you cant draw pictures with this wiki answer thing
in pratical applications it can be used to determine a ships bearing (location from a certain point, i.e lighthouse or dock) , inclined planes in engineering
dont worry trigonometry is very useful and theres a reason why its being taught
Check out these articles for a simple free tool and tutorial that will make trig simple enough for ANYBODY to understand!
http://www.ehow.com/how_5520340_memorize-trig-functions-losing-mind.html
http://www.ehow.com/how_5227490_pass-mind-part-unknown-sides.html
http://www.ehow.com/how_5428511_pass-part-ii-unknown-angles.html