a valid question when your new to MTs and have never handled one!
My answer is:
Differences are size/weight, tools, materials, price, warranty, and overall finish/quality.
Leathermans are popular and they make many fine models for many needs. 25 year warranty.
Victorinox is in my opinion the best quality MT, the fit and finish are excellent but not as many models/types, and they take forever to come out with new stuff. Lifetime warranty.
Sog makes a good MT, not as good a finish as the others but you can easily buy and switch out tools, the others need special tool(s) and even then it's not easy but Sog has the strongest compound leverage pliers. Lifetime warranty.
Gerber makes a fun MT, not as reliable as the others (the most complaints I've seen) but they are getting better and they have some smart designs. Lifetime warranty.
Many more too like; Bear, Paladin, Schrade, SwissTech and more. Also the Chinese clones! Don't forget SAK! (Swiss Army Knife) it's a multitool
So do some research, find out what you like, go to a few stores (like wally world) and feel them out in your hands or ask friends to show you theirs, narrow it down and if you still have questions we do our best to help you.
Good Luck and enjoy!