Mulch is a layer spread onto the soil or around a particular plant. The purpose of a mulch is to keep the plant's roots protected in winter, cooler in summer and to retain moisture, which is why a mulch maybe spread around a particular plant. A mulch may also be spread over a whole border to suppress weeds.
Mulch such as composted bark is often spread over weed suppressing membrane to disguise it. Weed suppressing membrane is very effective, just not very attractive.
The best mulches are organic such as composted bark, compost, and leaf mould. An excellent commercial mulch is Strulch, which is an excellent all round mulch.
It is very good at suppressing weeds, light and easy to handle, and over time will decay down into the soil. It also looks tidy on a border.
All mulches are best applied, ideally, after weeding and when the soil is not frozen, or waterlogged.
Inorganic mulches are often used as part of the garden design, coloured slates and pebbles.