What are the key rule differences between tennis and pickleball?

In Pickleball, serving is done underhanded and you only get one serve. The return of serve must bounce. Scoring is usually done as side out scoring, meaning you must serve in order to win a point.
 
Transcript: "Hey there. So pickleball looks an awful lot. Like many tennis out there on the court, but there are actually a number of rules that are significantly different between the two. I'm only going to give you a couple now and then I'll leave in a bunch more. As I answer, the other questions out there, let's start right off with the serve in Pickleball, serving as done underhanded, and you only get one not to serve. So no double-faulted. Secondly, the return of serve must bounce. So tennis players, no serving and volleying and lastly, one of the more confusing aspects of the sport of pickleball scoring is done as side out scoring, which means you must serve in order to win a point kind of like volleyball before 1998, if you can. Remember that there's a few exceptions where rally scoring is played, but most of the time, it's side out scoring where you must serve in order to win a point. Aunt. Thanks Gregg for the question. I look forward to getting you out there on the pickleball court."