Austin will have more details, but basically with JetClean and CClean, it removes your cache. Which does slow down your machine if you have a lot of it. However, some things are important. A local company ran it on my other machine and suddenly I couldn't log in to do my timesheets anymore because the domain disappeared and I didn't remember what it was. I also lost a lot of websites because I didn't have them bookmarked, I relied on cache.
Commercial products of Disk Defragmenter are said to do a better job than the resident Windows tool. I can't remember if JetClean and others do that or not.