In typical dungeons and dragons notation, a d100 is a hundred sided die. Normally this is emulated by rolling two ten sided dice and having 0/0, or 10/10 be one hundred. It's a fairly good system for determining raw percentages.

I have however at one time, seen an actual one hundred sided die. It was this weirdly shaped plastic ball, that contained sand. It was rather strange and hard to roll. It was about the size of a half dollar, probably a little bigger if memory serves. The point was to roll it, and it would magically stop on some number. It was weird and a freakish novelty for some RPG gamer somewhere. I never bought it, since it was a little over ten dollars for a piece of cheap plastic that would eventually break.

My advice: Stick with 2d10 for getting a percentage; although you could impress all of your friends with this large and strangely amusing device.

There are lots of ways to roll d100. As noted by JayBonci, you usually roll two d10 and use them as the two digits. Before rolling, you should say which die is the first and the second digit, so you need two different dice.

It is quite easy to find d10 dice with 00, 10, 20, ... on their sides, so you can use them together with normal d10 to roll d100.

Our club uses a simple convention to roll d100. We roll a d10 and a d20 together, getting the first digit from the d10 and the second from the last digit of the d20. There's no need to call the dice before rolling (as we always use this technique). It is also useful because quick and dirty dice packs often have only a die of each kind.

As someone who, in the good old days, was the proud ownder of a d100, I believe a number of comments are in order.

A d100 is huge - not something you want to carry in a pocket.

Once in motion, it keeps on rolling. (see below)

Numbers are tiny, which makes it very difficult to use.

They are easy to lose. I owned a d100 for a couple of weeks. During a RPG convention, I used it for demonstration purposes only.
It went...
...off the table...
...down a flight of stairs...
...after being reflected off a wall, down another flight of stairs...
...and lost from sight.
It then either disappeared into nothingness or (more likely) found a new owner among the hundreds of RPGers.

JayBonci is right - not that that would surprise anybody.
Unless you have spare cash, a set of d10s is a much better buy.

A d100 is a 'probability die' often used in RPGs like Dungeons and Dragons. Also often found noted as d% (percentile dice), it mostly clearly denotes a chance percentage coupled to a certain probability. A d% is rolled for several different occasions. A few examples:

-Looting a creature slain by tabletop knights often requires the d% to make out the actual loot

-Encounters with creatures are often governed by a d% roll. The higher the roll, the harder (usually) the encounter.

As previously noted, a d100 is mostly 'rolled' by the use of two d10's, each with different colors or sizes, where initially agreed upon what die governs tens and what governs units. Also previously noted, a physical d100 is rare and often large.

A physical d100 is an invention made by a man named Lou Zocchi; it is a spherical die upon which the numbers 1 through 100 are written. Though multiple attempts at d100s have been made earlier, the invention of this 'Zocchihedron' as it is called in 1985 has been lauded the birth of the first 'agreeably honest' d% die. Noted however is, that it still isn't as completely integer as two d10's. Truth be told, physical d% dice are a lot more expensive and mostly considered collector's items. I possess two, and I think of them like this, too. Other dice often used in RPGs are d3, d4, d6, d8, d10, d12 and d20, though also known are d5, d16, d30. There are even d7's and d24's.