2 main types:

1. CMOS sensor chips with lead attached back to nearby computer; or sometimes an aerial and wireless connection. PROS - Allows for nearly instant picture on screen. Relatively cheap to set up if only using single sensor.
CONS - The thickness of the sensors can be difficult for some patients and some situations. Sensors are very expensive to replace if damaged or if want multiples.

2. Phosphor plates.
PROS - as thin as normal Xray film and used in same manner. No leads. Sensor plates are very cheap, so can have multiples available (I have about 25).
CONS - much slower to produce picture (minimum 2 mins my staff can do at best resolution). Expensive setup with laser scanner, about 4-5X cost of CMOS.

Some people say you don't need as much radiation exposure with digital radiography, but from using my own system (phosphor plate), I don't agree you can reduce it too much without picture degradation. Have also seen Xrays from CMOS systems and poor quality noted too. I do a lot of RCTs and I want resolution as close to standard films as possible, so have increased scanning time of my laser above that of manufacturer's advertised time, and at same exposure length as with standard Xrays (0.27 secs).
If you're looking to buy, then I recommend the phosphor plate systems. But get a demonstration and play with the settings of exposure and resolution for a day. Make sure the computer software is also easy to use.