The Basics
You've found Moab on a map, and you want to go there. You have time off work and permission from the SO. That's the hard part.
Moab lives in a valley running north and south, with the Colorado River bookending things to the north. The valley was created by a fault, the most clear example of which can be seen driving by the entrance to Arches NP. The faulting, the river, and the mountains to the east create an amazing variety of riding, why you're going in the first place.
History
Moab was settled by Mormons back in the day, though natives were there long, long before. Many examples of rock art, from the pedestrian to the hidden, surround the town. After WWII, once the Cold War began, the US Government had an obvious interest in stockpiling Uranium, and Moab was the center of this monopolistic boom. The vast majority of the roads in the greater Moab area, including many classic rides, were created by the industry of the Uranium prospectors. Some struck it big, and for a brief period in the 50's Moab had more millionares per capita than any other town in the US. Once the government had a 300 year stockpile of Uranium, their subsidies were lifted and the boom collapsed. Moab slumbered until jeepers, bikers (motorized and non), rafters and tourists "discovered" the area in the early 90's. Growth came for good, and new hotels keep sprouting.
Orientation
The valley works to your benefit as a visitor, because most everything is on Main Street (aka Highway 191) which runs through the middle of town. Center Street is the main east/west street, and on this intersection is the Moab Information Center, a good place to park, use the bathroom (open buisness hours), and walk around investigating and gathering supplies. The Mormons have a brilliant system for laying out roads, and numbers grid using the cardinal directions. So, 200N 300E is two blocks north and three blocks east of Center and Main. There are some idiosyncracies and exceptions in Moab, but for the most part the system makes navigation easy for the newcomer.
Weather and Seasons
A general guide. As always, don't trust the weather.
Spring (March-May): Begins the influx of spring breakers and Coloradans escaping the snow. Lots of people, earlier every year. Hotels charge full rates again, restaurants reopen. Early March evenings can be cold, and snow is town is a distant possibility. Overall day temps are very nice, staying in the 60s (typically!) during March, getting into the 90s by late May. In any season in the desert, direct radiation makes a huge difference. Full sun and wind protection can raise the temps 20 degrees, and of course the opposite is also true. Beware. Jeep Safari is the week before Easter, and is hell-on-earth crowded.
Summer (June-September): HOT! It's a desert, and by mid-July 100+ is not uncommon. If you must, go and ride early, then late. Bring tons of water. Alternately, get up early and shuttle up into the mountains, and enjoy singletrack at 10,000'. By mid-September things get a little more reasonable.
Fall (October-November): Fall break, shoulder season, Cottonwoods and aspens changing, fall in Utah is very nice. October also has the most unstable weather of any month, with a week of sun alternating with a week of overcast, periodic rain. Sun is nice, but rain seldom lasts all day, and it packs down the sand. Post 24 Hours, things begin to settle down and get cold.
Winter (December-February): My fav. Cheap hotels, empty trails, and always an open pool table at the brewery. It usually doesn't snow in earnest until after Christmas, so warm gloves and a tolerance for shorter days can reward a T-day trip. By the new year evening temps well into the teens are the norm, so camping is for the dedicated.
Bike Shops
Bike shops in Moab tend to be good. The usual foibles apply across the board, but they're more likely than most to have a selection of 2.5" tires and new hydro hoses. All have a better-than-average rental fleet. That's all I'm sayin'.
All should have the Lat 40 "Classic" Moab Trail map, which I think every visitor should buy first thing.
What Bike?
Most anything in Moab can and has been ridden on anything, up to and including unicycles. That said, the average rider will want dual suspenion with a fair amount of travel, and large volume tires. 2.5 Nevegals for the more aggressive, 2.4 Mutanoraptors for the more XC inclined (for example). The many rocks and ledges also tend to make pinch-flats somewhat profligate.
Food
Yes. Lots, some good, some not. Below are my favorites. At 7pm on a Saturday during the season, everywhere will have a horrendous line.
Coffee: Mondo's, in the mini-mall thing just south of the Info center.
Moab Brewery: Nice sit down food. Burgers, good salads. South of Center on the west side of Main.
La Hacienda: So-so Mexican. On Main near the north end of town.
Moab Diner: Best breakfast, any time of day. On Main, just south of the main concentration around Center.
Accomidations
For hotel/motel I like the Red Stone Inn. A little south of the main action, but cheap, nice, with microwaves, a hot tub, and outdoor bike stand with hose.
Decent cheap camping can be found down Potash Rd (north and then west of town), or out River Rd (aka 128) north and east of town. The sites at the base of Porcupine Rim (~4 miles east on 128) are nice and only $5, but go fast.
Water
North of town, 100 yards down 128, is pipe coming out of the rock on the south side of the road. Matrimony Springs. I've always drank it straight, and filled up a jug there 3 days before I got hitched on the spur of the moment, so in my experience it works. If you're road tripping with your boy/girlfirend, you've been warned.
South of City Market, Gearheads Outdoor store offers unlimited free filtered water during store hours (usually until 9pm during the season).
And last, DRINK LOTS. One 100 oz bladder is not enough. Dehydration kills.