Les Gaillands is the closest crag to Chamonix and as such is immensely popular. But don't give up if you arrive and find a swarm of schoolchildren on the easy routes on the main crag, because there's a lot to choose from here, plus there's more rock just out of view to the left.
Another reason for Les Gaillands' popularity is that it's one of those something-for-everyone crags--unless you hate bolts--with varied difficulty, multi-pitch options, fantastic views of Mont Blanc and the Bossons Glacier, and a buvette at the bottom if you want to grab a drink or snack.
The rock? Gneiss. The main crag (Grand Gailland, Echelles, Petit Gailland, left to right) faces south and has no shade until late afternoon, and mosquitoes come out in the evening. Shade abounds at the other crags, though (Viperes, Forestiers, Chausseurs, Frendo, Piscou, continuing left up the hill).
There's a bus stop at Les Gaillands (Line 9), but you can walk there from Chamonix Centre in 20 minutes or so. Follow the signs for Les Pèlerins--it's a much nicer (and safer!) walk than the main road out of town. Walk through Les Pèlerins, join the main road and pass a small hotel and bar, Le Vert, on your right. The crag is in the park just beyond, on the right; you'll easily see it from the road.
Wide range in climbing difficulty and well bolted for sport. Mostly single pitch in the main area with some longer routes available. A bit hot and sweaty in the sun. Close to the bus/train stations and well within the carte d'hote zone (free public transport if staying at a hotel in the Chamonix valley).