The much anticipated (or feared?) Steam Summer Sale has begun today and will run through to July 22, bringing with it offers on large games and small. The format of the Sale appears to be following that of more recent sales, concentrating on headline sale titles - for instance BioShock Infinite today - and adding around a dozen smaller titles as part of their 'Daily Deals', running for 24hrs. Updated every eight hours are the Flash Sales, which often have deeper discounts on other titles and are voted on by the community.
More games, such as Torchlight II @50% off, are running throughout the whole of the sale period. There doesn't yet appear to be a repository of all these titles yet unfortunately so you'll just have to dig them out; however it's often best to wait for them to go on a Daily Deal anyway as these tend to have deeper discounts.
There is no specific theme this summer, but unsurprisingly Valve are pushing their new Trading Card achievement system. The idea is that by playing specific games and completing certain achievements you'll be able to craft trading cards, which can then be redeemed for vouchers, held in your collection or sold on the Steam market. You also have the chance of bonus cards by voting in the community sale poll and for every $10 you spend on Sale items.
The main Steam website is currently up and down like a fiddlers elbow so you may be better off waiting for a while before grabbing what you want. If there's something you're looking forward to why don't you let us know in the comments.
UPDATE: Also, don't forget that other etailers will continue to have their own deals over the sale period. Green Man Gaming for instance have their own offer for BioShock Infinite coming in slightly less than Steam's (even without additional vouchers which are common on the platform), and nets you a key which is activated on Steam anyway.