Types of mulch...
Leaves are the oldest form of mulch. Naturally when trees go dormant, they drop their leaves in order to conserve energy for the winter months. The leaves protect the soil around the tree from getting too cold. Most gardeners don't like the look of tree leaves because it looks unfinished and sloppy. A benefit though is that during the spring and summer, the leaves compost (break down) and put nutrients back into the soil. It's beautiful I tell ya!
Bark mulch is most commonly used in garden beds and around trees and shrubs. It can also be used next to walkways and in between stone pathways.
Straw Mulch is most commonly used in vegetable gardens. Straw has a very tight knit nature so it blocks sunlight from hitting the soil and growing weeds. Straw also decomposes relatively slowly as opposed to newspaper shreddings and grass clippings.
Grass Clippings mulch is most commonly seen on lawns. Many people bag their clippings and just throw them away. They're essentially throwing away money! Many lawnmowers have a "mulch" setting where the clippings get shot out the side of the mower and sit on the lawn. This process is known as "grasscycling." The clippings break down back into the lawn and provide nutrients while controlling moisture levels.
Newspaper mulch has been becoming more and more popular. Newspaper shreddings absorb water very well and help regulate the moisture levels in soil. As of late, mulching with newspaper hasn't been widely accepted due to the idea of ink getting into the soil. Now-a-days "most" newspaper companies have been converting to organic based ink and dyes that are safely biodegradable part of their "green" initiatives.