Manual flash units come in two types and are known as 'Non Dedicated' flashes as they can be used on just about any camera equipped with a hot or cold shoe. (a cold shoe has no flash contacts)
Manual only. These are the most basic flash guns available. They usually offer a couple of power settings (eg: full power, 1/2 power etc) and you have to fully configure the flash by hand.
Auto flash units are a more advanced manual flash, these units generally have 2-8 power level settings and also an 'auto sensor' circuit that controls the duration of the flash (ie it's total light output) thus ensuring a more accurate level of flash illumination on the subject. This is achieved by moutning a sensor in the front of the flash (usually by the flash mounting foot) that detects the light output and when it has reached a suitable level for good exposure it turns off the flash. This gives a crude by quite effective method of control.
Dedicated flash units:
TTL (Through The Lens) flash control is considerably more advanced than manual flash systesm.
These flash units rely on the camera to measure the actual light generated by the flash falling on the film plane to determine correct exposure. Once the correct amount of flash illumination is reached the camera instructs the flash to terminate the flash output.
OEM (Original Equipment Manufacturer) TTL flash units are dedicated to their own brand of camera and often to specific models inside the brand, eg: Nikon, Canon, Pentax etc). There are Generic TTL flash units that will work with specified brands, such units are made by the likes of Achiever in HK and often will work with 2-3 brands of camera.