a form of saying hello or rather a way to let someone know you're around.

Literally, to poke someone. Typically this is done over an IM client.

Part of the old school emote family (yeah, before lolcats).
Like anything enclosed in asterisks, *poke* is performing the action of poking the person you send it to.

Largely replaced, in modern context, with "/e", "/me" or "/em" (emote) preceding the action.

Typically, usually a middle aged "pedophile", in the works of only talking to 10 to 16 year old's by networking, "Internet" slang.

As he will, "*Poke*" he strokes, until the other person figures they themselves are oblivious that there is in fact, a pedophile cybering, with he/she.