Buy a good basic book about knitting (or borrow from a friend). Stitch and bitch has some good information. Also, some of the weights of yarn have different names in the US and in Europe. You could probably do a quick search on Aran yarn and find some different kinds and you would see the gauge, which would help. Okay, i tried it and aran yarn looks to be a worsted to heavy worsted weight yarn, does that help?
What I do is look at the yarn I have and compare the weight (worsted, dk, fingering, aran, bulky, etc.) of the yarn to what’s called for in the pattern. The gauge of the yarn I use is usually somewhere in the range of the gauge for the pattern, because they may not use the same needles.
If the gauge of the yarn is somewhere near the gauge of the pattern, then it’s time to knit a swatch and see if you can come up with the same gauge as the pattern that you want to knit.
So if the pattern says the gauge is 18 st per 4 inches, and your yarn says 20 per 4 inches, it’s likely the same general weight and is worth swatching.
If the pattern says 18 per 4 inches and your yarn says 24 per 4 inches, then it’s probably too thin to work.