How do I use Hashtags?

[JD]

I'm just a Noob to the online community, and have completely no idea on how to use them.

I fear I might end up looking like a troll if I used them with no idea how to use it.
I think it has to do with relevance, where you put tags n it to allow other users to find it
That's what i thought at first when Nokia on Facebook told us to put their hashtag in for pictures relevant to their theme, yet it doesn't make any sense.

[Damon]

Hash tags are used on Twitter and Facebook to link you to other statuses/Tweets containing the same hash tag. You can only use letters. No numbers or punctuation. If you clicked on a hash tag like this: #STL, you would find all statuses or Tweets with that same hash tag in it. Within a reasonable amount of time. Not sure how long they go back.