|2ply||Baby||2.25 to 3.5mm||Baby clothes|
|3ply||Fingering||3 to 4mm||Baby clothes,|
|4ply||Sport||3.5 to 4.5mm||Baby clothes,|
|Double Knitting||Double Knitting||4 to 5mm||Children's clothes,|
|Light Worsted||toys, adult sweaters|
|Aran||Fisherman,||5.5 to 6.5mm||Adult Sweaters|
|Chunky||Bulky||6.5 to 9mm||Adult Sweaters,|
|Extra Chunky||Super Bulky||9mm-12mm||Bags, Rugs|
Yarn comes in a wide variety of thicknesses, and it can be quite daunting when you first start crocheting to know which yarn to use. Most patterns will specify a yarn, and give an approximate gauge / tension size, usually how many stitches and rows over a 4 x 4 inch square. (Tension is the traditional term used in knitting in the UK, but the US term is gauge).
It's important to check your gauge / tension before starting a project, to make sure you have the correct yarn and hook size, especially when making something that needs to fit, like clothing. Once you've been crocheting for a while, you'll be able to play around with different weight yarns and not just stick to the yarn weight specified in a pattern.
The names for different weight yarns vary between the UK and USA. Bear in mind that even yarns in the same weight can vary a great deal, eg Paton's Diploma Gold DK is much thinner than say, a budget acrylic DK like Paton's Fab.
I've included the recommended hook sizes for each yarn, although again, this varies depending on what you are making. For a bag you would use a smaller hook to create a dense, tight stitch, but for something drapey like a shawl or scarf you would use a larger hook. A crochet pattern will always tell you which hook size to use.