Cravez's explanation was pretty good, although there were some errors, particularly with the translations of things.

For example:

QUOTE:
e.g Neko ga misu o Nomimasu
(The Cat is drinking water)

If you'd like to say "The cat is drinking water," you'd use:

QUOTE:
Neko wa mizu wo nonde imasu.

Water is "mizu" and not "misu," and in order to represent an ongoing action you need to use the -te/de form of the verb followed by "imasu." The dictionary form of the verb here is "nomu," which means to drink, and the -de form is "nonde." Therefore, it becomes nonde imasu.