Mulch - the single best thing you can do for your garden! As for "type" it depends on the circumstances. As for "quality", clean and weed-free are about the only criteria. What you need to keep in mind is that with mulch you get a double whammy - moisture retention and weed suppression. You can use a lot of things for mulch - the lowest maintenance is fist-sized rocks, but I think they're ugly. I like nice soft stuff like shredded cedar bark. I also use wood chips if I'm not too concerned about nitrogen depletion. The wood chips you get from a tree service or municipal yards are kinda big and chunky. The small chipper/shredder I rent yearly from the Home Depot makes nice, small, uniform chips that make a beautiful mulch. You can use hay, straw, seaweed, compost, manure, grass clippings (dried, please), newspaper, cardboard, or just about anything that suits your fancy. If you're growing tomatoes, Lee Valley offers a red plastic mulch which is supposed to reflect red light up at the plant and promote fruiting - I'll leave that one up to others.... there's also a black plastic mulch which heats up the soil faster in the spring.....in short - you have a lot of choice.