What is this Dota you speak of?
Defense of the Ancents was originally created in Warcraft 3 by the mapmaker Eul, and was based off of the Starcraft custom game Aeon of Strife. When WC3: The Frozen Throne was released Eul stopped updating the map, which then caused a flood of different varients to appear across battle.net. Among them was DotA Allstars, which was created by Guinsoo, which became main version that we have today. During Guinsoos time, Pendragon came along and made dota-allstars.com in reponse to the need for a main dedicated website for the growing community. With version 6.01, Guinsoo passed authorship off to Icefrog, who started adding new heroes, items, mechanics and fixes, and still runs the game to this day.
Guinsoo and Pendragon would later go on to work at Riot and help create League of Legends, while Icefrog and Eul would go on to work at Valve and are now working on Dota 2.
Dota 2 is the sequel to that popular Warcraft 3 custom game that has since spawned an entire genre with the likes of League of Legends and Heroes of Newerth. If you have played either of those games then you have a basic idea of how things work.
Gameplay remains true to the original Dota, but takes advantage of everything the source engine and Steam have to offer. Besides better graphics, this means things like reconnecting, being able to spectate any match currently in progress even while waiting in que for matchmaking, and no longer having this happen to you
New to the game and want to learn? The following links will get you started. Dota has a massive learning curve, so you wont learn everything here, but it should be enough to get you on your feet.
(Credit for this list actually goes to Gully State over on Gaf.)
Second (Warning, these are long.)
"Hey, I'm freaking amazing at LoL so I should be good at this too, right?"
How to spend starting gold
Warding for map vision
Not exactly. While at their core they are similar, there are some some key differences that make the meta/feel/pace different. Some of these include things like the way carry heroes work, denying, gold loss on death, towers behaving differently, runes, a larger map, a str/agi/int based stat system, more active/useable items, and more.
Heroes & Items
There are currently 108 heroes in Dota, and while all of them will be coming over to Dota 2, only about 3/4s of the hero pool is available in the current beta build. Currently we get about one new heroes a week, with Valve sometimes surprising us with either more or none at all.
Current hero pool: http://www.dota2wiki.com/wiki/Heroes
Curious what heroes are on their way? http://www.dota2wiki.com/wiki/Unreleased_Content
Other Useful Links
Announcement art (also in the valve store as posters)
(This OP is a work in progress, more info will be added as more people are able to get their hands on the game.)