Angles
Where any two lines cross each other, they form four angles. These angles are parts of a circle (as you can see from the green circle segments), so, like a circle, they have to add up to 360 degrees. If the two lines are perpendicular to each other, then the four angles are each a quarter of the circle, so they're each 90 degrees. We call that a right angle. Squares and rectangles, because their sides are perpendicular to each other, each have four right angles.
If the two lines that cross are not perpendicular to each other, but skew, then they'll form four angles of which two will be acute - smaller than 90 degrees - and two will be obtuse - bigger than 90 degrees. But they'll still add up to 360 degrees altogether. In this picture, angles 1 and 3 are acute, while angles 2 and 4 are obtuse.
Main Geometry page
To find out more about geometry, check out these books from Amazon.com or from your library:
