What are No-Intro sets?
No-Intro is a group dedicated to collecting ROMs of older video game systems for the sake of preservation. Their main goal is to collect complete and accurate collections of old video games. This means no hacks, bad dumps, etc, only clean ROM files accurate to what was commercially released. The name "No-Intro" came from the early days when they used to rip out annoying "scene" intros that were sometimes injected into dumped games. If you want more information you can visit the No-Intro website.
Difference between No-Intro and Goodsets
The most common complete ROM sets you will encounter on the internet are known as "GoodSets". While these aren't bad, they have a slightly different goal and are usually several years outdated. Goodsets basically collect together as many known dumps as possible into one set. The good, the bad, hacks, multiple dumps, all there. No-Intro sets only include one good ROM for each region or revision of a game (usually one US, one Europe, and one Japan). In addition, No-Intro sets are updated frequently as new or more accurate ROMs are found. Basically what you get in a No-Intro set is equivalent to what was on the original cartridges.
What are the numbers next to each set?
After the Name of the set there are parentheses and numbers. This is the date when the set was last updated in Year-Month-Day format. For example a set with (20101126) was last updated on November 26th, 2010.