Reformatting essentially will wipe your drive (the partition) completely clean. You will have NOTHING left on the drive/partition you reformatted.
If you are going to reformat, make sure you have up-to-date backups of all documents, pictures, music, emails, logs, saved-game files, etc. Make sure you have all installation disks you need because you WILL need to reinstall everything.
By the way, most major operating systems have options to reinstall the actual OS without reformatting...basically replacing all the system files and nothing else. I've seen those fail before, so if your system seems corrupted, beware, but in almost every situation that won't happen.
On the more positive, because you're wiping everything clean, all viruses, spyware, adware, other malware, and the general clutter that Windows seems to accumulate....will be GONE.