Sassie, intervals are a type of speedwork you'll often see on schedules, usually marked confusingly as "Intervals: 8x800m" or something similar.
We can take 8x800m as an example. What that means is that the runner runs 800 metres fast followed by a slower recovery run - that fast-slow pair counting as one "rep" or repetition - then repeats that process seven more times. Therefore, "eight times 800m".
Of course, the unwritten catch is that "8x800m" doesn't mean "800m times eight" - it means "800m fast, times eight". So, if your recoveries were the same distance (a fairly common convention) you'd actually run 1600m times eight, in total.
For example, last Tuesday I ran an 8x400m session. I ran slowly to my local park (to warm up) and ran to a section which I knew was the appropriate length. Then, I ran the 400m to the other end fast, and jogged back to the start; ran to the other end fast, jogged back to the start... etc. After eight times of doing that I was more than a little knackered - intervals are nothing if not tiring!
Intervals are most often done in areas where you know what distance you're covering, which will mean either a track or a section of land that you know well.
Hope that helped :)