P25 (Project 25 or APCO-25) is a suite of standards for digital mobile radio communications designed for use by public safety organizations in Canada and North America. P25 radios are a direct replacement for analog radios but add the ability to transfer data as well as voice, allowing for a more natural implementation of encryption or messaging. P25 radios are commonly implemented by dispatch organizations, such as Police, Fire, Ambulance and Emergency Rescue Service, using vehicle-mounted radios combined with walkie-talkie handheld use.
For us HAM's, P25 is a digital voice protocol that we use over RF and employ though RoIP (Radio over IP).
Other examples of digital voice protocols are DMR, D-Star, Fusion, NXDN and Tetra.
There has been a resurgence in the interest and use of P25 here in Canada.
As more and more surplus equipment becomes available, HAM's are snatching them up and re-purposing them for Amateur Radio use.
A large part of the surplus equipment is due to the U.S. "re-banding" or narrow banding of their radio spectrum.
Using digital protocols means that you can get twice as many "channels" of conversation and data transfer into the same bandwidth as standard analog FM radios.
I won't get into posting TONS of links related to Amateur Radio and P25.
Rather, I will give you ONE link ... to Google ... that will search for ... Amateur Radio and P25