The idea behind mulches is simply putting down a layer of material to help the soil retain moisture during a growing season, and to control the growth of weeds.

If you choose an organic (plant-based) mulch, you also encourage beneficial bacteria and earthworms, and at the end of the season, you can turn it in to the soil.

Some Commonly Used Mulches

• Garden Compost - best of the lot, if you have enough!   Get that heap going!   You can never have enough compost - if I had, I'd be spreading it with gay abandon to a depth of 2 to 4 inches almost anywhere.   Reality limits me to spreading a good layer around fruit bushes in spring to control weeds - and encourage the worms.
• Straw (seed free), hay (loads of seeds), grass cuttings (may have weed seeds) - need to be laid in a layer 3 to 6 inches deep.   Said to attract earth worm activity, and they do improve the organic content of the soil when spaded in at the end of the season.   A deep straw mulch is especially useful around strawberries to stop the fruit being splashed by mud.
• I have not seen chipped bark used as a mulch in a vegetable garden, only in flower gardens - probably because few of us have a cheap source, it would take a long time to rot down, and I suspect it harbours slugs.
• Inorganic black plastic sheet laid over the bed, with holes cut through for your onions, your potatoes, or your sweetcorn.   Good at warming up the soil, and controlling weeds, but make sure the plastic carries rain water TO the plants, rather than away from them!   On my own beds, plastic would be like a pitched roof so it wouldn't work; and it would be a wonderful hiding place for slugs and snails.

Weed-clearing mulches

• Old carpet (furry side down), a thick layer of newspaper or cardboard (laid wet, and covered over with a bean/pea net, thin black plastic, or clear plastic wrapping from electrical goods shops), or again thick black plastic sheet, all held in place with bricks and stones, or a thick layer of grass cuttings.   Leave in place for at least 2 or 3 months - and longer to kill the more persistent weeds.   On carpets, learn from my experience - AVOID artificial fibres and rubber-type backings.   At least the natural fibres "self-compost" eventually.   With artificial fibres and backings, the weeds eventually grow through the nylons and rubbers - and you are left with a truly unholy mess to clear, including a layer of undigestible gobbets of gunge.
• In theory the thick mulch excludes light, and the weeds die; as the weeds rot down, they add to the soil fertility.   It works best if you use it in spring or early summer, when the weeds are in growth mode.
• I'd use this in two situations.   First, if you have just taken over a large plot or garden - so you'd dig the part you want to use this year, and mulch in spring/summer the part you want to prepare in the autumn for use next year.   And second, if you've hired a rotovator to clear a new plot, but you only want to cultivate part of it immediately.