Flash types

i-TTL

i-TTL works by firing a pre-flash before the mirror is raised when you take a picture. The meter evaluates this preflash, as well as other information (Aperture, ISO, selected AF point, AF distance, Flash exposure compensation) and determines an exposure. All TTL flash with the D50 is i-TTL. the flash then fires at the selected power level after the mirror is raised and the shutter opens.

TTL (analog 'Through The Lens')

TTL flash reads the exposure via reflected light off the film to a flash sensor in the mirror box (or the camera meter in the case of the F3). It is more accurate than a pure pre-flash based system, but due to the high-reflectivity of Digital sensors, it doesn't work well for digital. TTL meters only for the flash exposure, ignoring the ambient exposure, this is the mode to use when shooting with the flash as the main light.

TTL-LB (i-TTL Balanced Fill Flash) (D50 default)

TTL-BL balances the ambient and flash exposures so as to use the flash to fill in shadows and bring out the subject. It works fairly well but seems biased to use the flash as the main light. It's better to use TTL-BL with the Flash Exposure Compensation dialed down a stop or so (-1). The on-board flash on the D50 does TTL-BL only, the mode is selectable with the SB-800 or SB-600.
via: Nikon D50 flickr group