The Annual ticket is a Gold Card, which gets you 1/3 off most journeys in the South East e.g. if you take a leisure trip not covered by your Season ticket.
You can get a Season for any period of 1 month or greater, if you can't afford the initial investment of an annual then it can be best to buy them in 5 week blocks (starting Mondays and finishing Fridays - but varying that to take into account bank holidays, annual leave etc)