This is basic Rubik's Cube theory.
The Basics
In a cube, there are 12 edges, 8 corners, and 6 centers. Therefore, there are three basic types of pieces in a puzzle cube; the edges, the corners, and the centers. The larger a cube is, the greater proportion of the cube's total surface pieces are center pieces.
Orientation refers to the polarity of a piece. Orientation however, is not the same as permutation. Permutation is how the pieces are arranged relative to eachother, orientation refers to how the pieces are flipped or rotated while staying in their spot.
So, a spinning top would be constantly changing it's orientation while not changing it's permutation  a moving elevator would be changing it's permutation while maintaining it's orientation, and a bowling bowl moving towards the pins would be changing it's permutation and orientation.
Anything beneath this line has not been made simpler for mainstream readers. You can go ahead and read it anyway, but without diagrams or further explanation, (coming soon!), you may be easily confused as I was while reading it.
Legal moves
Permutation
Every possible algorithm performs an even number of swaps.
As it turns out, every cube state reachable by legal moves can always be represented by an even number of swaps, and at the same time cannot be represented by an odd number of swaps (the two are mutually exclusive).
To understand why this is so, we need to realise that each legal move always performs the equivalent of an even number of swaps. No matter how many moves you perform, the number of accumulated swaps will therefore always remain even.
For example, consider the turning of one face by 90 degrees:

→ 

The new corner state can be obtained via 3 swaps (swap C1/C4, swap C1/C3, swap C1/C2). Similarly, the new edge state can be obtained via 3 swaps. All together, this is 6 swaps which is even. Therefore, no matter how many moves you perform, always an even number of swaps will have been performed.
Since exactly half of the conceivable permutations are even and the other half are odd, only half of the cube's permutations (ignoring orientation) are reachable by legal moves
Edges
Every possible algorithm flips an even number of edges.
To establish this, it is necessary to decide on a frame of reference for correct edge orientation, regardless of where an edge is positioned on the cube. The most common frame of reference is to say that an edge in the wrong position has correct orientation if, when it is moved to its correct position using only the left, right, top and bottom faces, it would have correct orientation. Using this frame of reference, it is easy to see that any move on the left, right, top and bottom faces will always flip zero edges, which is an even number. The only remaining faces are the front and back faces. In both of these cases, a 90 degree move will flip all 4 edges, which is again an even number of flips. Therefore, it is never possible, using only legal moves, to flip an odd number of edges.
Corners
Every possible algorithm turns the corners so that the number of turns clockwise is divisible by 3.
Let's say that a corner has orientation '0' if it is twisted the correct way, it has an orientation of '1' if it is twisted clockwise, and it has an orientation of '2' if it is twisted even one step more clockwise (this is the same as just twisting anticlockwise).Once again, to establish this, it is necessary to decide on a frame of reference for correct corner orientation. Notice that every corner either belongs to the top or bottom and therefore each corner has one of its stickers with either the colour of the top face or the colour of the bottom face. The most common frame of reference for correct corner orientation is to say that a corner has correct orientation if its top/bottom sticker is facing up or down. If it is facing in any other direction, then this corner is not correctly oriented. Using this frame of reference, it is easy to see that any twist of the top and bottom faces will not change the orientation of the corners, and therefore the total orientation will remain exactly divisible by 3. For any of the 4 sides, a 90 degree turn will twist two corners by orientation 1 and the other two corners by orientation 2. The total change in orientation is 1+1+2+2 = 6, which is divisible by 3. Therefore, no matter how many legal moves you make in a row, the corner orientation will always remain divisible by 3.
Overall
For every 2*2*3=12 combinations, 1 is actually reachable without taking the cube apart.
Symmetry
Two different cases can be the same if the colors, reflection, or rotation is different, but everything else is the same.
Group Theory
Rubik's cube can be viewed as a mathematical group, where each element of the group is a permutation. As a group, it has the following properties:
 Closure
 If P_{1} and P_{2} are two permutations in the group, then P_{1}P_{2} (i.e. the result of P_{1} followed by P_{2}) is also a permutation in the same group
 Associativity
 Performing P_{1} followed by P_{2}P_{3} is the same as performing P_{1}P_{2} followed by P_{3}.
 Identity
 There is a permutation in the group in which no pieces are moved.
 Inverse
 For each permutation in the group, there exists an inverse permutation which has the reverse effect.
Rubik's Cube also has a number of subgroups, each having these same 4 properties.
The property of groups that is perhaps most interesting to cubists is closure, which means that each operation within a particular group will take you to another element in the same group. This means that if you choose a subgroup and use only operations from that subgroup, then your cube will remain within that subgroup.
Conjugates and Commutators
A conjugate is in the form of XY(X) where (X) is the inverse of X.
A commutator is in the form of XY(X)(Y).