A Bit of Westeros History: (Earliest to Present; all from just watching the TV Show)
*White Walkers*: Their existence was established the very first episode of GOT. They appear to thrive in very cold weather so they probably live very far North of the Wall. They have sent scouts down towards the wall (accompanied by wintery weather) to kill men and animals. (This gives heaviness to the words of The North: "Winter is coming".)
This is their way of 'recruiting' troops. They are necromancers and every man or animal they kill they can turn into a reanimated corpse called a 'Wight'. Wights will do whatever the White Walkers instruct them to do.
They appear to have been around earlier than the landing of the First Men. It's unclear what food source they use to subsist on. (Speculation is that its all the dead men and animals they have cut to pieces. In S1E1 the body parts of the butchered Wildlings disappeared. One of the ones intact became a Wight child.)
They have glowing blue eyes and white hair and appear to be stronger and move faster than men. Any reanimated dead (Wight) have glowing blue eyes like the White Walkers. Wights are highly flammable and can be destroyed only by setting them on fire.
Samwell Tarly has concluded White Walkers can be killed only with obsidian daggers or arrow points (called Dragon Glass).
It's unknown how many White Walkers there are or troops (Wights) they possess, however one should bare the following in mind:
1. They have been around longer then men on Westeros and for thousands upon thousands of years.
Reanimation haults the decay process of a body. The number of dead that have been recruited could be in the tens of thousands easily.
2. The Wildlings burn their dead so they are doing their part at keeping the White Walker Wight Army population under control.
3. Living human babies are being used to generate new White Walkers.
4. White Walkers appear to have camouflaged keep way up North in the Lands of Always Winter.
5. They have an alter and group of 13 White Walkers (the lead one has a head with thorny protrusions) that are used to generate newborn white walkers.
A species of deciduous trees found all over Westeros. The leaves and sap of weirwoods are blood-red, while the smooth bark on their wide trunks and wood are bone white. Most weirwoods have faces carved into their trunks. This was done by the 'Children of the Forest', and is now done by the free folk as well as other descendants of the First Men, such as followers of the old gods in the Seven Kingdoms praying to heart trees in godswoods. The wood never rots and lasts thousands of years (cut or uncut.)(One can be seen in the Godswood at Winterfell where Ned Stark sat under one polishing his blade, 'Ice' in the first Season.) Andals cut down all the weirwood trees south of The Neck of the North because of religious reasons. The Throne of the Eyrie in the Vale is made from a dead one.
*Children of the Forest*:
Were at Westeros before 'The First People' and last seen during the invasion of the Andals. They were small Druidic creatures (not men) who had magical capabilities. (Speculation: It's likely some of these creatures bred with the 'first men' creating men with magical powers. SEE BELOW)
(Ned Stark and Maester Luwin spoke of them.)
*The First Men*:
Ancestors of people of Westeros that invaded Westeros (using Dorne) 12,500 years ago from somewhere in Essos. They held 'The North', which the Andals were never able to take when they invaded Westeros. They built the wall 8,000 years ago and the 'first men' that got stuck behind the 300 mile long wall became known as the Wildlings (Free Men.)
*Free Men (Wildlings)*:
Burn their dead so the White Walkers cannot recruit them as Wights.
Ancestors of people of Westeros that invaded Westeros 6,000 years ago (using the Vale) from the west coast of Essos.
Over time the peoples started mixing together through settlements, marriages and breeding.
Speculation: This may also explain why some Westeros folk have magical capabilities like Warging (Bran, Jojen), Greensight (Bran) and being impervious to fire or heat (Daenerys).
The 7 kingdoms and the houses that established and live in them have origins from all this. (The Crownlands, Dragonstone Island and the Riverlands are additional territories.)
Throughout the show characters refer to some houses as being of Andal or 'the First Men' descent.
This is why the King of the Iron Throne is titled: King of the Andals and the First Men, Lord of the Seven Kingdoms and Protector of the Realm.
Game of Thrones: Threads at the TV Critic