Disassembling items is the easiest way to get materials for crafting other items. The items or materials you get from disassembling an item will vary depending on the type of item you are trying to disassemble. For example an item such as a Glyph will give you runes which can be used to make other consumable items. Trying to disassemble a weapon will give you metals which can be used to make other weapons. Then you have Talismans, which give different things depending on the type of Talisman and the list goes on.
Ultimately the main thing you will need to forever know about disassembly is the fact that it is the main way to acquire materials for crafting other goodies. Disassembling items and selling the materials you get is also another means of getting money from an item. Sometimes you can get much more than vendor price if the item's materials are something people really want (Tank/Healing Pure materials for example). To get started with disassembly hit "Y" on your Keyboard to bring up your Crafting Menu. From there place the item you'd like to disassemble into the Results window.
What comes up in the window is as follows.
Note: The order that the items come up in all Disassembly windows is the order of the materials you would have to put into the window in order to craft the item. Keep that in mind when disassembling items you might want to craft in the future!
As you can see the button at the bottom says Disassemble and I have a list of all the materials in the crafting window that I will get for disassembling this item. The QL(Quality Level) of an item directly affects the type of materials you can expect from disassembling an item. QL10 green items give you Sacred materials which are one step below Pure, the highest level material. Before I close out this guide with some lame opinion I'd like to mention an extremely important fact about Disassembling items that is best known early.
The Tradeskill materials that most QL10 green items disassemble into are worth way more than you could ever hope to get from the vendor for the item itself. In otherwords I just answered the question everyone will ask at one point "Is Disassembling items worth it?"
That's really about all there is too disassembly though. It's a bit tough in The Secret World since the game doesn't have a complete list of items you can search through to craft. It's a lot more complicated and a lot more oldschool I think.