If you get a DSLR with a DSLR housing, the housing will have a connector to the hot shoe to an external strobe. Connection to the type of strobe usually is limited same mfg as housing unless an adapter is used. The ability for the camera to perform the metering of the strobe will depend on the strobe's compatibility with the camera mfg.
If you get a digital P&S camera that has a hot shoe with an Ikelite housing, the housing will usually will have an hardwiring capability to an Ikelite strobe, thus triggering the strobe the above way.
If you get a digital P&S camera using the camera's own housing, it usually won't have hardwiring of the hot shoe to an external triggering wire. So you have to rely on optical triggering of the external strobe by your camera's built in flash (usually via a fiber optics cord).
The simplest of the optical triggered strobe is a manual one, where there is no metering, you just adjust the strobe's strength based on distance to subject and trial and error. The strobe usually will have pre-flash cancellation feature which when your camera shoots pre-flashes for metering purpose, the strobe will ignore them and only fire off when the main flash fires. Inon, Fantasea works like this.
The second type is an auto-exposure strobe where the light sensor is on the strobe itself. It triggers the same way as the manual strobe, but when it fires off, the built in meter measure when enough light has bounced back from the subject and shuts off the strobe. If you place your strobe closer to the subject, the meter will be closer too and underexpose the picture. I think Sealife is an example of this.
The third type is a strobe that emulates the camera's flash metering with pre-flashes by also firing off flashes when the camera shoots off pre-flashes. Thus the camera thinks the strobe's pre-flash flashes are its own flashes and will fire off a final flash with strength based on the pre-flash's return brightness, and that final flash's strength will also determine the final flash's strength on the strobe. If you place the strobe closer to the subject, the exposure should not change since the camera's meter will perform the metering. What the strobe basically do is emulate the duration of the camera's pre-flash duration, only it being stronger. Example of this type includes Inon, Sea & Sea, Ikelite AF35(?)
Some strobes allows for triggering without a fiber optic cable. It's light sensor faces in the direction of the camera's flash and triggers when it sees the flash go off.
The forum usually recommends the cheapest Inon (S2000) or Sea&Sea (YS-01) of the third type because it could work from low end cameras to higher end rangefinders to SLR (if the flash can pop up in the housing). Some strobes of the third type also accepts hardwire cabling from SLR housing, thus could work with non-transparent SLR housing.
