Normal vs Safe Boot?

Hi all,

So I seen the words "Safe Boot" a lot on the internet,

but I've never exactly found the difference between normal and safe booting. (I've booted into SafeBoot once, on a Win7 desktop, just found that the background is black, and everything else looked like the normal boot UI)

So whats the difference between the two?

I heard that going to safeboot is better when a virus is present. But why is safeboot better for when there are viruses/malware present?