Most of the time, you see cirrus clouds very high up in the sky, looking thin and wispy, like someone pulled a bigger cloud apart into little bits of cloud. Cirrus (SIR-russ) clouds look thin because they are made of ice crystals, not water drops. They form where it is high enough to be cold and freeze the water drops into ice.
Sometimes you might think you see cirrus clouds when it's really only the trail of an airplane, half blown away and spreading across the sky. But either way, if the sky is blue with only these thin cirrus clouds high up in it, you can be pretty sure it's going to be a nice day.