A teleconverter transforms the lens into a lens of longer focal length by whatever the conversion factor is (usually 1.4X, 2X or occasionally 3X). Meanwhile, the physical diaphragm does not change in size, so if you have a 1.4X converter, the effective f:stop is one stop less than what it says on the aperture dial, and with a 2X converter you lose two stops, and the DOF will be the DOF for a lens of the focal length you get with the converter at the aperture calculated with the converter for the applicable subject distance and format.
If you are metering through the lens, you don't really need to think about this--just use the converter and do what the meter tells you. If you are using an external meter or a non-TTL auto flash or manual flash, then you need to include the exposure factor. If you are not metering through the lens, you might also want to test for transmissive light loss with the converter, which can sometimes be as much as half a stop, which is important if you shoot slide film.
Converters also reduce image quality somewhat by magnifying the flaws of the main lens, but sometimes the tradeoff for convenience is worthwhile.