"PA," even though we still commonly use it, is really a rather outdated term. It originally was an abbreviation for "Public Address," like the announcement speakers in the airport or at a baseball stadium—not exactly the sound quality you want for your music. "Sound reinforcement" sounds so much better, as do modern speaker cabinets.
From portable setups to arena-sized concert systems, the technology of sound reinforcement has evolved in a major way over the years. Today's cabinets are lighter, louder, and better-sounding than ever. New materials and computer-aided design have revolutionized how sound is delivered at all budget levels.
Today, when considering new cabinets, one of the major questions is whether to go with active or passive cabinets. Passive cabinets are what were traditionally thought of as PA cabinets—speakers in a box. A passive cabinet needs at least one external power amp, more if you want a bi-amplified system, with an external electronic crossover and separate power amps for the low- and high-frequency program material. Passive systems require a little more setup time and care, and require carting more gear, but compensate by being just a little more flexible, especially when something fails, since you can just move it to a different power amp or swap out a speaker fairly quickly and get back to the show.
Active cabinets have power amps and electronic crossovers built into the cabinet, so you can hook directly from the outputs of your mixing board to the input of the speaker. They can also be daisy-chained, so it's easy to expand your system to whatever size you need for a particular venue. Many newer active cabinets use lightweight neodymium drivers and equally light Class-D digital power amps to keep weight down. Some of these new cabinets can provide 1000 watts or more of power in a cabinet that weighs less than 40 pounds. They make for a fast, easy setup and easy custom configurations for different venues.
Also readily available are lightweight, powerful subwoofers—a big help with music that absolutely requires huge bass. Available both in active and passive versions, modern subs are also smaller, lighter and louder, packing more sound into less cubic footage than ever.