What are "Tags" ???
 

Please explain to me how to use them and what for?

I used to think that for example, if I tag a thread with: best, antivirus, 2013

then if someone was searching for the best antivirus for 2013 he would see my thread but I think I am wrong

so are they even necessary? If they're not, then I won't waste any more time filling them out for each thread I create

please advice in detail

thanks

Check out vbulletin.com as they use tags:
https://www.vbulletin.com/forum/tags.php?tag=antivirus

Now click on a thread and go to the bottom where people have 'tagged' the thread.

I'd be interested in an explanation also as to what good they are, what purpose they serve when you can search on words within any post already? What is the intent of "tags" and are they really worth worrying about?

I think it really depends on the site. I have a music related site and the users use them to tag the torrents as "rock" or "country" or "pop", etc. The torrents are all automatically tagged with the name of the band, so they can quickly click the tag and get a listing of all the torrents for that band, instead of using search and getting all the threads where that band is mentioned.