Wall clock time is the actual amount of time taken to perform a job. This is equivalent to timing your job with a stopwatch and can be affected by anything else that the system happens to be doing at the time.
User time measures the amount of time the CPU spent running your code. This does not count anything else that might be running, and also does not count CPU time spent in the kernel (such as for file I/O).
CPU time measures the total amount of time the CPU spent running your code or anything requested by your code. This includes kernel time.
The "User time" measurement is probably the most appropriate for measuring the performance of different jobs, since it will be least affected by other things happening on the system.