The CCD and CMOS sensors in most “normal” cameras (DSLR, Bridge, 4/3 etc) are sensitive to visible light, as well as ultraviolet (UV) and infrared (IR). During the manufacturing process, a filter (e.g. a “hot mirror” filter) is laid over the surface of the sensor which effectively absorbs the UV and IR to record images in “natural” colour.
If this filter is removed, the sensor will again record UV and IR. It can either be left in this state, when the camera is said to be a “Full Spectrum” camera, or specific filters can be placed over the sensor to selectively absorb visible, UV or IR, so that the camera can record either UV or IR.
Most types of digital camera can be converted: DSLR, Bridge, compact etc.
Recommendation: If you are interested in trying both UV and IR photography, then go for a "full spectrum" conversion. If you know that you only want to shoot UV or IR, then go for a specific conversion.
A full spectrum converted camera can be converted back for "conventional" photography by using a Hot Mirror filter on the camera lens. This is really useful if you need to shoot a visible light "control" image as well as the UV or IR image. One example is supplied by Kolari Vision (see links)
A number of companies can carry out the conversion, mainly in the UK or the US. I have listed a few in the Links section of this site