This is the modern networking protocol used by Steam versions of CS 1.6. Unlike the older Protocol 47 (used in v23/v26 builds), P48 allows players to join the vast majority of current online servers, which have largely phased out P47 support to improve security and Steam integration.
Often includes a "switcher" or emulator (like dproto) that allows the client to communicate with both P47 and P48 servers.
This refers to the specific engine build. The v36 update focused on stability, fixing several "buffer overflow" exploits that previously allowed malicious servers to crash a player's game or execute code on their machine.