I'm still not good at imagining what yarn in a picture online will look & act like in person, but here's what helped me.
I read this forum and when Ravelry came along, explored that...and listened to podcasts and such.
I heard and read the names of many yarns. Then I had a chance to go to a local yarn store (LYS), and lo and behold, there I found labels on yarns and found myself saying, "Oh! So THAT's Noro Kureyon!" or "So THAT's what Cascade 220 is like!"
I mainly just started paying attention...I fondled yarn at the yarn store, and noticed the softness of bamboo and whatever.
I saw some Rowan Kidsilk Haze and wondered why anyone in the world would want to knit with it, though I hear it raved about now and again.
So..the more you explore, the more you will learn. If you're like me and a visual person, seeing (and feeling) in person will help. I try to buy a little something everytime I go to the yarn store, but really I'm just learning about what I see in patterns and magazines and on Ravelry and here.
In the meantime, pay attention to gauge. I now can realize without even looking that a yarn that gets 3.5 stitches to the inch on size 10 needles is going to be a chunky yarn---and one that gets 7 stitches to the inch on size 3's is a pretty skinny little yarn...just from looking at ball bands and paying a little attention to patterns and such.
I did ask a lot of questions in the beginning, but really, I found that a lot just has to be learned by experience (or math, but I try to avoid that whenever possible! :))