Welcome to SB.
The first stage regulator usually attaches directly to the tank valve, and reduces the tank pressure (say 200 bar or 3000 psi) to the IP (Intermediate Pressure) of roughly 125-160 psi. The first stage will typically have one or two high pressure ports, which are connected to the tank side of the pressure regulating mechanism, so that you can hook up things like pressure gauges (on hoses) to measure how much is still in the tank. Low pressure ports are for everything else, like breathing (i.e. second stage regulators) and BC or dry suit inflator hoses.
A second stage converts the IP pressure to ambient, so that you can breathe. Connects to the first stage through a hose. It's the thing you actually hold with your mouth. The one you use most of the time is also called your primary second stage, or primary for short.
An octo (aka 'safe second') is a backup second stage. Useful if you have certain types of problems with your primary, or wish/need to donate air to someone else. Some people like octos with less features (i.e. less expensive and/or less things to potentially go wrong), while others prefer identical high-performance regs for both primary and secondary.
BTW, it would help if you filled in your profile, so that others can get a better idea of how much detail to include when answering questions. From the way your questions were worded, I'm guessing you're new to diving, so please excuse me if that's not true and the above seemed at all like talking down to you.
