Often when starting out in crochet all the different yarn weights can be very confusing, especially when they are called different names in different countries around the world so below is a list of all the different yarn weights as well as a list of what projects they are best used for, at least in my opinion.
It's worth noting that although most yarns will display a recommended hook or needle size on the label this isn't a hard fast rule to follow. It is perfectly acceptable to use different hook sizes to create different effects. Often using a bigger hook will make a looser drape and vice versa, a smaller hook will create a tighter knit. This is especially useful to note if you want to adjust to your own tension. Don't be afraid to be creative and not follow the rules!
Starting with the smallest and following the convention of US / UK / AUS ...
Lace-weight / 1 ply / 2 ply
Known as lace-weight in the US, this is one of the thinnest yarns you can find and it's one I've never actually used personally as I am way too impatient when it comes to crocheting, but if you want to make really tiny projects then this is a great yarn weight to use.
Sock or Fingering or Baby / 2-3 ply / 3-4 ply
Very similar weight yarn as lace, just a little bit thicker. Really lovely for delicate projects such as crochet or tatting.
Sport or Baby / 4 ply / 5 ply
I like to use this as an alternative to a DK / light worsted weight yarn, especially in socks. It can take a while to work up, but I like to use a slightly thicker hook than recommended such as a 3.5mm or 4mm crochet hook to create a beautiful soft drape.
Light Worsted / DK / 8 ply
This weight yarn is probably the one I use the most as it's just the right amount of being not too thin but not too thick, so I use it for all types of projects, especially amigurumi and socks.
Worsted / Aran / 10 ply
This weight yarn is more popular in the US and is just slightly thicker than my favourite the DK. It's a good alternative for those who want that extra thickness and projects that work up quickly, but don't want too much chunkiness.
Bulky / Chunky / 12 ply
Often yarn labels get confusing when it comes to bulky and super bulky yarns and is probably one of the main reasons people have some problems with sizing when I state using a bulky yarn as in the US this can sometimes be a little thicker so bear this in mind. This yarn weight however is great for blankets and scarves as it works up really quickly! Use with a 5.5mm or 6mm hook, but you can go bigger with an 8mm for that extra drape if it feels too stiff.
Super Bulky / Super Chunky / 14 ply
One up from a size 5 standard bulky this yarn oozes that extra chunkiness. I rarely use this yarn as it is very thick to work with, but it would certainly make for a very quick blanket or scarf! Just make sure to use a good 8 or 10mm crochet hook, if not bigger.
Fashion Yarns
Finally there are yarns known simply as fashion yarns. These types of yarns don't fall under any particular yarn weight and can range from anything from a fingering yarn to a super chunky yarn, but will usually include distinctive features such as bobbles or hand-spun textures. Usually the label will give an indication of what hook size to use, but you can have a lot of fun with these yarns experimenting with all types of hook sizes!