Circles, squares and diamonds designate the degree of difficulty of slopes and trails at a ski resort. True beginners should stick with circles. You will have WAY more fun if you progress incrementally. More experienced skiers and snowboarders should also pay attention to designations.
Here is a quick explanation on how the trail signs are determined.
In general, approximately 25 percent of the trails are designated green, about 50 percent are blue, and about 25 percent are black. If a mountain is steep all over, the green-circle trails can be a challenge for novices. They are the easiest trails at that resort, but they may be tough for someone who is learning. (Image via Mountain High)
A few resorts use the symbols in combination to show even more degrees of difficulty. Here is an example: