D10

It's a die. With ten sides. Pretty simple concept.

It's in the shape of a pentagonal trapezohedron. You can stand it on its point and spin it like a top.

Your d10 dice are not this good-looking.

Rolling a d100 With 2 1d10s

Some Systems require you to roll d100s frequently. The only problem with that is that actual 100-sided die are fucking stupid. So, instead you roll a 1d10 twice. It's just like rolling 2d10, except you do not sum them up. Instead, you use one die to denote the singles digit and another die to denote the tens digit. Since the events are independent, there are exactly 10*10=100 possibilities when you do this, all equally likely. You now have a method of rolling 100 different numbers, as desired.

Some d10s have two digits per side (see above) to make differentiating your digits easier, but you roll d100 with any two d10, provided you specify beforehand which die is the tens and which the ones.

Reading it off is simple. Did you roll a 20 and a 4? That's a 24. A 90 and a 1? 91.

Where some people get confused is when they roll two zeroes: a 00 and a 0. Obviously you rolled a 100. Why? Because the alternative is a 0 which you can never roll with any other die. That, and any table you're rolling on assumes a roll from 1 to 100, not 0 to 99.