Here is a great article on what happens right before a child typically learns how to crawl and the various styles of crawlers:
ZERO TO THREE: Steps Toward Crawling
The only thing I would add to this article is some other types of atypical crawling:
-segmented: the child crawls but it appears slow, and not fluid as if every movement of the arms or legs have to be thought out. This can be a sign of a variety of problems including neurological
-bunny hop: the child crawls with the hands and then hops forward with both feet simultaneously coming up under him, just like a rabbit does. This can be a sign of neurological, muscular, or orthopedic problems
-bear crawls and can't bend knees- this is when the knees are extended vs. bent, it can be nothing to worry about unless that is all the child can do. It could be due to muscular tightness, orthopedic, or neurological problems
These atypical patterns in addition to the ones listed in the article (e.g. using one side only) may resolve after the child has been crawling for a couple of months. If they do not, this should be discussed with the child's pediatrician to consider a referral for therapy and/or a specialist (neuro or ortho)..
It is important that children crawl, but not mandatory. I have a 16 year old niece who skipped crawling and began walking at 8 months. She climbed furniture by 10 months, and went on to be a high level competitive gymnast. She takes advanced classes at high school. So, although she has no gross motor or learning problems, skipping crawling can be an indicator for that...not always, but can be sometimes. Crawling helps the brain learn to use the two sides of the brain together, develop arches within the hand muscles for fine motor control, and promote depth perception (eyes). Maybe this is why my niece's handwriting isn't exactly the best...not awful, but not great either!