Can someone tell me what constraints are, and how do they work? And what are they used for?
Thanks!
Google is your friend, aswell as the search feature.
http://www.blender3d.org/documentation/htmlI/x7484.html
Thats from an early version, but it gets the point across
I’m a Linux user, so I am very familiar with google 
But I forgot to check it. Sometimes im just slow…
Probably the most common use that I have for constraints is modeling mechanics. Lots of the things that I’m modeling are steam-engines and the equipment that is driven by them. This means that I must create models which exhibit reciprocating motions, with the correct motions around the correct hinge-points and shafts.
Usually, the most effective way to model this is to start by keyframing the rotation of the object: in other words the engine does not actually drive the shaft, but the other way around. This means that one end of the crankshaft must follow the center of the crank-pin on the wheel, and the other end must be constrained to follow the linear path imposed by the piston. Constraints allow me to do that. Blender solves the constraints and in so doing calculates the correct orientation and position of the crankshaft (and perhaps other connected parts), frame by frame.
Constraints, and “parenting” and “empties,” are therefore a tool that I use fairly constantly. Mechanical animations are always tedious work, but this relieves at least some of the tedium.