In the usual case, متي is used as a question tag with the meaning of "when". while عندما is used with the meaning of "at the time of".
But there are some cases when they are both used as time conjunctionsthe, where difference between both is very subtle and could even be replaceable. The main difference would be that متي is often used to consider the time in the future (near or far), and rarely used with the past. While عندما is used with the past, but could be equally used with the future as well.