Mulching serves several purposes in the garden. It keeps most annual weeds from germinating. It is less helpful with large, perennial weeds - and if it worked on them, we really wouldn't want to use it in the perennial border since it would also keep our garden perennials from coming up. It also feeds the soil and plants as it decomposes and it just makes the garden look neat and 'finished'.
This lovely mountain is wood chip mulch. It would be expensive to buy in the quantities that we use, but we have been lucky in that tree trimmers who need a place to dump their day's 'work', occasionally will be working nearby and will empty their trucks here. The city of Athens will also bring us their excess. We don't use them right away, but rather leave them for a year or so to mellow a bit before we use them near plants. In large open spaces, new chips are all right. They know that we don't want anything with Black Walnut since some plants are sensitive to that, but what we get is usually a mix of hard and soft woods. If you're buying bagged mulch, look for some that is called Hardwood Mulch because it will last a lot longer that pine. Avoid Cypress. Also avoid stuff with dye in it. Besides being ugly and unnatural, it just can't be good for the plants.
This is an area we recently mulched with wood chips. They gradually rot and will need to be replaced 3 or 4 years from now, but in the meanwhile, they're keeping the weeds down and feeding the soil as they decompose. Our gardens used to be hard clay, but in many places I can plant small things by hand without even needing a trowel. Never thought my soil would get that good, but mulching really helps.
This is our other big source of mulch material. The City of Athens, in the fall, brings out the bagged leaves that people put out for pickup. We never quite use them all up, but the remains become one large compost pile. They also bring us shredded leaves which are dumped in a big pile and which I really like, but they are harder to use in that I have to shovel them into a cart to bring into the garden where the whole leaves are in bags. I tend to use the shredded stuff near small plants or in more formal settings and the whole leaves in more woodsy places. Leaves last a year or two before they need to be replaced. They also do a really good job of keeping weeds down and feeding the soil and plants.
This little Variegated Sage was mulched with shredded leaves.
A more unconventional type of mulch is this chipped limestone which we use around cactus. We rarely have any weeds in this bed - thankfully, since it is so hard to weed. I remember, when I was a little girl and we would go to the beach in the summer, that people used white rocks in their flower beds. I would assume it was for the same reason. Obviously, this is a pretty permanent mulch.
This is probably the ultimate mulch for acid loving plants - pine needle. We will never have enough of this and, unlike in the South, it isn't available to buy here in the North. We do have a bit of pine woods where I can gather some and some comes in the leaf bags (why, of why, would someone get rid of them) so we can usually find enough for the plants that really appreciate it. It lasts for a few years also.