The Horcruxes are the way that Voldemort extends his life. When you kill a Horcrux, some of your soul is also destroyed.
Horcruxes can be anything (though they’re typically inanimate objects). They don’t have to be magical or valuable… they just have to be meaningful to one of the people who made them. It’s never explained why Voldemort chose those particular objects, but it was likely because each of them had some significance to him, and he wanted something he could keep with him at all times.
It’s interesting that each of these objects was either owned by someone important to Harry or was connected to someone important to Harry (Dumbledore). That makes sense because Voldemort wants Harry to destroy them by destroying himself… which is why he tried so hard to kill him when he was a baby.
Lord Voldemort has made seven Horcruxes: Harry Potter’s wand, Tom Riddle’s diary, Slytherin’s locket, Ravenclaw’s diadem, Hufflepuff’s cup, Nagini the snake, and Voldemort’s ring.