Are you asking if you can use apps that are made for your computer on your iPhone? The answer is no, unless there is a version that's made for your phone, like iTunes and safari. Same app, but different version.
No the other way around, apps made for my iphone to use on the computer. Lets say I downloaded my "to do" list, or "my diary" for when I log information to remember at doctors appts ect. I want to work on that on my computer then sync to my iphone. Is that possible?
There are some third party apps that you can purchase for to do lists etc. I know with MobileMe you can synch back and forth with calendar and contacts however at this point I don't believe to do lists synch.