"defragment" a mac?

[BeachBum]

When I had a PC and it started to run slow, or run out of memory I would defragment it. I think that it just arranged things in chunks so that the free memory was all in one place.
I keep getting a notice that my hard drive is almost full. I plan to delete my iphoto library after I move it to a external storage.
How do I defragment after that? Do I even need to?

[wendibird22]

I have a mac. I don't know if there's a way to defrag, but my IT guy at work always has me go and "repair disk permissions." You can find this in your Disk Utility (Applications/Utility/Disk Utility). Have it check for and repair permissions. I've run this several times after I delete software or delete a lot of files (be sure to empty your trash first).