OK - disc brakes - 3 versions:
Mechanical
Single pot hydraulic
Double pot ( or more ) hydraulic.
Mechanical ones usethe same brake cable that a V-brake would use. Generally cheaper and performance generally on a par with a good set of V-brakes. You can get more advanced ones whose power is better than V's, but at that price for them, you start to enter entry level hydraulic prices. Generally, with a mechanical you generally have 1 pad side that moves under cable pull, and the other pad side static. the static side pad ( normally the spoke side ) can have its distance from the disc adjusted by means of an allen key, so you need to run it quite close to the disc ( which can cause scuffing ) . The moving side then gets pushed against the disc, and then against the static pad to clamp. This slightly bends the disc as it does so and I found that in some cases this lead to uneven wear of the pads ( a little like toeing in brake pads would cause uneven wear ) and that you did need to fettle with the distance to get it right. The upside of mechanica discs is that they are simple and cheap.
Single Pot Hydraulic.
Basically the same as mechanical really. the hydraulics move only one side the pads and the other remains static. Power will be better than that of mechanical V's, but you still have the issue of slightly bending the disc. One of the most common used to be Hayes 'Solo' 9's
Dual Pot ( or more ) hydraulic.
These are the real deal. Instead of 1 moving side and 1 static side, you have a caliper piston on both sides pushing both pads against the disc in an even manner. This means that the pads clamp the disc without bending it and equal force and wear on each side. There are a ton of different versions from the various manufacturers and its a hugely debated topic. Even the base model versions from the likes of Shimano nowadays are outstanding compared to V-brakes ( personally speaking from experience ) .
Each make also tends to have a slightly different feel to the way the brakes work as well. Generally I found that Hayes brakes have quite an ON/OFF feel to them. The power ramps up really sharply as you pull on the levers. At the other end of the scale, Hope brakes in comparison have a softer feel to the them where you need to pull the lever a longer distance ( not necessarily harder pull, just longer travel ) to get the same power of braking. This is good when your wanting to feather the brakes and kill speed in a variable controlled manner as opposed to an all out stop. Inbetween those is Shimano. Not as harsh as Hayes, but not as mushy as Hopes. ( dont get me wrong about hope though .... pull that lever and the anchors are well and truly on )
You've already decided, but I'd suggest, that it may be worth waiting and buying an aftermarket upgrade to disc brakes. PLaces like
CRC and the like often do deals on full hydraulic systems ( dual pot calipers ) and these are likely to be a better long term fit.