MRU = Most Recently Used (from pages, links used by you)
Tracking Cookies are placed on your machine by most sites you visit. Some are just a nuisance, some are adware/malware.
Let Adaware remove them.
Critical Objects are those things that 'could' cause a problem for your computer by causing anything from popup ads, allowing a worm/virus/trojan to be installed on your computer.
Depending on your surfing habits, it is a good idea to scan frequently to keep your box as clean as possible for best performance.
