A post office box (or just PO Box for short) is a service offered by many of the world's mail carrying services
. Formerly referred to as lock box
service here in the USA
Mail that is sent to the PO Box address (invariably "PO Box nnnn" (or, occasionally, one or two letters)) will arrive at a post office, usually has its own unique zip code for the box bank (e.g., 92836 is the zip code for the Fullerton, CA Commonwealth station PO boxes adjacent to the Fullerton Train Station), and is almost always sorted by 10:30 AM - generally earlier. Each office has a box clerk as well, who is analogous to the routed mail carriers that otherwise deliver to your home, as available.
There are a few advantages to taking a box as an option - you get your mail earlier than most people, for one thing, and it also provides for a little bit of anonymity. That aside, they're also cheaper than a box at places like Mailboxes Etcetera.
The disadvantages, while few, are simple. For starters, any item requiring your signature (certified mail, for instance) generally require you to proceed to the line to check with a clerk; if you habitually get your mail after hours, this becomes a little inconvenient.
Also, you generally can't get private courier deliveries at a PO box, but sometimes a post office will accomodate you. Check with the box clerk or the local postmaster - occasionally you can get places like UPS to "mail" your package to your box by having the item shipped to the post office, but that also costs extra in postage.
Parties who receive inordinately large volumes of mail (IE, the daily volume of mail exceeds the capacity of your PO box) might subscribe to caller service.
Cost on box service is highly variable, and is paid every six months. The smallest boxes can be as little as $9, and the largest boxes might be as much as $330. In cases where there is no carrier delivery of mail, you become part of Group E, and get a free box of whatever size is available. In addition, keys cost $1 for a deposit for the first two (refunded on relinquishment of keys), $4 for each additional key copied, and a lock replacement if necessary costs $11.