Distinct from what I'd call art DJs, who boast deep and unique collections and specialize in remixing and blending tracks together, commercial DJs have a straightforward job: play familiar songs that will get everybody dancing. "Dancing Queen" by Abba. "Come On Eileen" by Dexy's Midnight Runners. "Wonderful Tonight" by Eric Clapton. If you've been to a wedding in the last few years, you can probably come up with the rest of the playlist yourself. I'm sure a lot of DJs work in both realms, just like plenty of live musicians play in wedding bands to support their soul-liberating artistic projects, aka "real" bands.
My brother recently began working for a company that, among other things, books commercial DJs. Curious, I asked him about the type of gear these DJs use. Playback gear varies, from massive CD changers to turntables to--occasionally--MP3 players, and of course they all own or rent high-wattage PA systems, mixing boards, lights, and microphones for conducting toasts.
But here's something that surprised me: most of the DJs he books use Web software called DJ Intelligence to help create a commercial Web site. There's a module that will let customers see the songs in the DJ's collection (with links to third-party lyrics sites), and create playlists (as well as do-not playlists). Other modules let the DJ create a detailed planning form for the event (remember that there will be two best men for the toast!), keep track of guest requests during the night, and--most useful--accept payment online. Personally, I was most interested to see a list of the top 200 most-requested songs over the last 12 months, which is compiled automatically based on online requests into the system.