Hi sneakachu,
Whilst items are in quarantine, they are unable to affect your system.
Deleting a quarantined item will indeed remove all traces of the contents from your computer. The reason that it is considered best to retain quarantined items for several weeks, is that the items removed may include files essential to either the operation of your system, or a program on which you rely. You may find that you need to reinstate a particular item to get your system functioning properly again.
Not every single item detected is actually harmful - sometimes there is the occasional "false-positive", which is what this section of the forum is for:
False PositivesHope that answers your question.
Regards,
Spike