Most painting/animation programs represent the animation as a sequence of images of identical sizes (a frame). Depending on program, each frame has an individual duration (this one lasts 1/10s, this one lasts one full second, etc), or they enforce a single duration for each frame - in this case you duplicate a frame to make it 'last longer'.
Read up on traditional animation, as it has a lot of education material : This thread links the most well-known terms and books.
http://wayofthepixel.net/index.php?topic=16571.msg150326#msg150326What kind of things/subject do you want to learn animate ?
Tiny stuff like animated smileys ?
Larger action game sprites ? (from NES mario to metal slug?)
Even-larger fighter sprites (like street fighter - 100 pixel high and up)
For any "large" stuff, a principle is generally to get the "dynamic" and timing right, before pixelling any details : If the subject is very large you'll sketch and animate with the outline. If the subject is smaller (so the thickness of the outline is a problem, it's annoying to erase and clean lines every time you change something), you draw solid-colored shapes, until the animation plays right.