Defragment is when files break into non-contiguous segments. If a disk is empty, a file can be in contiguous blocks. When you start deleting existing files, you get "holes" in used space that can be used for new files. But if the holes are not the full filesize, they will span into many of the "holes". In short, fragmented.
But OSX automatically defragments as part od invisible housekeeping. Third-party products tend to introduce errors instead of fixing problems.
Much-used systems sometimes have too many holes to allocate large disk-sections for BootCamp after several years of delete/re-use. But if your system is new you should be fine for years.
Is a big issue with Windows that does not autodefrag, but not with OSX.
Apple's position on defragmenting: http://support.apple.com/kb/HT1375
The article is outdated but still accurate. Defragmenting has only become less relevant since then, and will cease to be an issue altogether when hard disks become obsolete.
A more important concern is your perceived need to do this. If you are having problems, explain the symptoms.