How do solid-state hard drives (SSD) compare to hard disk drives (HDD)?

I’m wondering how to compare the speed of a solid state hard drive to a traditional, spinning hard disk drive. In particular, I’m thinking about ordering the SSD upgrade to my new MacBook Air.

Traditional hard drives speeds are measured in RPMs but solid state hard drives, by their very nature, don’t revolve. So what’s the equivalent? How much faster is SSD? Please link to any applicable benchmarks.