Mine are most often random - except with my first pregnancy. With that one, I wanted apple juice and chocolate milk early in the pregnancy (not at the same time). I wanted apple juice so desperately at times that I would actually have to leave work at one point and walk across the street to the store to get some (luckily my FIL was my boss). Those cravings for juice were so bad that I would keep two gallon jugs in the refrigerator and I would finish them in a couple of days. I also craved salads for most meals. Those three cravings stayed with me from start to finish in that pregnancy.
My second pregnancy, cravings started at the end of the first trimester and I wanted cokes all the time until the end. I would have a few random cravings with that one (I want a Hershey Bar today or I want Twizzlers, etc.).
This time around, I craved Froot Loops like crazy in the first trimester - I went through two boxes in a week! But that tapered off after a couple of weeks and now I just get the odd craving every now and then. I do want orange juice a lot every morning, but I was sick with a cold that has dragged on for three weeks, so I think it's more an attempt to soothe my cough than to appease the baby.
Most of mine have fallen into the "moods" category, but the apple juice cravings for 8 months in my first pregnancy are what I imagine it's like to need a "fix" of something, because I seriously had to have it or could not function. It was crazy.