While pretty close, most descriptions of load average misreport one essential piece of information. Load average is the average number of processes either in the running state (often marked with an R in programs like top) or in the waiting for disk state (often marked with D). That means that even if your system is hardly using any CPU but several processes are reading from disk your load can still be quite high. This makes sense, as it's an indication of system load, not just CPU load.
Note that most unixes treat NFS and other network filesystems as disk, so when you unplug your laptop while reading files over NFS, your load will spike because every process trying to access the now unreachable files will add one to the load average.

Load average is usually displayed as three numbers, the first being averaged over the last minute, the second over the last 5 minutes, and the third over the last 15 minutes.