The answer is that mulch is a blanket - it is any material that you use to cover the bare soil in your garden beds. It could be compost, bark, fallen leaves, pine needles, a composted combination of sawdust and manure, even rocks or crushed glass (I do not recommend the last choice in particular, but I have seen it done) - anything that covers the soil while allowing water and air to penetrate.
Mulch is usually applied in a layer 1-1/2 to 2" deep. This depth will help hold moisture in the soil so you won't have to water as often. It will also do a good job of keeping weeds under control. I believe that weeding without mulching is a waste of your time. When you have weeded and loosened the soil, you've created a perfect place for weed seed blowing by on a spring breeze to land and germinate. A thick layer of mulch is not as "germination friendly" as your freshly worked soil, so it keeps new weeds from getting established. If there is already weed seed in the ground, which is often the case in areas that have been weedy for a long time, it has trouble germinating if it is smothered by a good layer of mulch.
|Heather bed before weeding|
1) They didn't put down enough mulch. A mere half inch is not enough.
|Heather bed after weeding and mulching with Steerco|
OK, now you know why you should use a mulch. So the question is: which one? A lot of people use bark, but I don't recommend it. Bark is not composted. After you spread it out over your garden beds, it will start to break down and as it composts itself it takes nitrogen from the soil to complete that process. That means there is less nitrogen available for your plants to use and they end up stressed. Notice the leaf color on plants sitting in a sea of bark. The leaves are often yellowish instead of deep green - and that's because they lack nitrogen. To keep that from happening, use a mulch that has already been composted.
(That said, I do like bark as a place holder. If you have a spot overrun by weeds that you plan sometime in the future to turn into garden space, weed it well and put down a thick layer of bark. It will keep the weeds down and when you are ready to use the space, you can plow the composted bark into the soil. Thick layers of coarsely chopped bark also make fine garden paths between raised beds.)
That narrows the choice down to compost (homemade or commercial) or products like Steerco and Gro-co. Steerco is a combination of composted sawdust and steer manure. Gro-co is composted sawdust and municipal sludge (in other words treated and composted sewage). Some people have no problem with Gro-co, others are creeped out by it. It is completely safe. The only possible objection I might have is that there might be a higher (but still very small) concentration of heavy metals in it.
My preference is Steerco, sold in Seattle by Sawdust Supply, the company that provides all the soil and mulch for the Northwest Flower and Garden Show. I've used countless yards of this material in the past 20 years and have always been happy with the result. It is already composted and contains microorganisms that perk up tired garden beds. It helps add structure to sandy soils and loosens clay soils - provided that you make it part of your annual bed clean up and maintenance. It does all the things you want a mulch to do, plus it has no odor and it looks great. It makes the humblest garden look professionally done.
On top of all that, Steerco is a bargain. A bag weighs somewhere in the neighborhood of 75 lbs. and costs about $5, including tax. Steerco is also available by the yard. You can pick it up yourself or have it delivered.
If you need to figure out how much mulch, bark or topsoil you need for a project, here's a handy chart.