What does a firewall really protect my computer against?

i have a firewall and a virus scan and virus protection but is a firewall and a virus protector the same thing?

What does a firewall really protect my computer against?windows

Firewall protect you pc against intrusions and fraud......Antivirus protect you against viruses,malawares,spywares,trojans.

you need at least a good firewall and good antivirus,use also a standalone spyware program and run it from time to time to insure your pc is clean.

If you are behind router that include NAT function or firewall,so you don't need any firewall software even the windows firewall..u can disable it.

What does a firewall really protect my computer against?antivirus download

no. not usually (depends what company you are using). a virus protector will scan your computer viruses, worms, trojans and other malicious hardware, check your emails for virus, ect.; while a firewall will prevent programs from downloading on their own (and depending how good/annoying your firewall is) it will even ask you to confirm the programs you are either downloading or starting up.

example: the vista firewall will always ask me if i am sure i want to download a certain program. and when i want to run a program, it will usually ask me if i trust it.

hope this helps, if not, send me more details:)
no,firewall blocks hacker intrusion while anti virus prevents MOST virus/trojan/worm infections