It is an ALT coin, a fork of the bitcoin chain happening sometime tonight. If you have your bitcoin private keys you will be able to claim BTG on a 1:1 basis after the fact.
Should I expect the bitcoin to go down? Thanks!
Definitely not much. Bitcoin Gold's community support is very poor, and it's trading volume and market cap are going to be extremely low, atleast for the time being. Thus, this should not affect BTC's value much, maybe it will go down a few 100$-250$ or so, but this is supposed to be very short term. Bitcoin Gold will only emerge as a pump-and-dump altcoin.