what is difference between last_worker_time and last_elapsed_time in DMV sys.dm_exec_query_stats?

Question

what is meaning of last_worker_time and last_elapsed_time in DMV sys.dm_exec_query_stats and what is differnce between them?

when I fire below query

    SELECT TOP 20 
qs.last_worker_time, qs.last_worker_time/1000000 last_worker_time_in_S,
qs.last_elapsed_time, qs.last_elapsed_time/1000000 last_elapsed_time_in_S
FROM sys.dm_exec_query_stats qs
order by qs.last_worker_time desc

I get reult like below.

On thing i notice was either both are equal or elapsed time is more than worker time.I would like to unsderstand significany of both so it also might help me in performance tunning.