The battery isn't charging

I'm MacBookPro user. For some time I have problem with battery. I noticed this for the first time when I downloaded many .doc (MicrosoftWord) files, opened it and after this action the battery stoped charging even though it was wired (The computer shows that it's wired but also shows the communicate that the battery isn't charging). Also the computer shut down once spontaneously, and once turned on at night. The next day I removed all dowloaded files ant the battery started charging again. But It's is still not charging normal way. When I turn the computer on it shows (for example) 91% loaded, then I log in and then It shows 100% loaded. Also from time to time it stops charging and then starts charging again.

I know that some PC users also have the same problems with battery in their mobile computers, and can only use them plugged in, because the battery isn't charging.

What's the problem?