Many DVRs are compatible with both NTSC and PAL standards. NTSC standard is predominately in North America and PAL in Europe. The PAL and NTSC standard actually refer to the method used to transmit color. The PAL standard actually requires 2 NTSC decoders to display video (one for each line alternatively) while the NTSC standard only requires one. The NTSC standard is supposedly less accurate in color display, but more efficient in the use of resources. In general, the DVR can be set to either decode NTSC cameras or PAL cameras, but not a combination of both at the same time. If you order a DVR in a package with the security cameras, then you shouldn’t have to worry about the setting or compatibility. If, on the other hand you purchase your cameras from one country, and the DVR from another, then you definitely should make sure that the DVR is compatible with the cameras. Check the standard of the cameras (NTSC or PAL) and the standard of the DVR. Remember that you cannot mix and match the cameras.
Also, keep in mind that just because you are in the USA does not mean you cannot have a PAL DVR or PAL cameras, or because you are in Europe does not mean you cannot have NTSC cameras or DVR. In actuality, you only need to be sure that the cameras and DVR are both compatible.