A set of values drawn from a population. The sample MAY be larger than the population if it is taken with replacement. Usually, the sample is smaller than the population because the population is very big (has a lot of values in it - e.g., the height of every male in Canada).
E.g., Imagine a population with two values in it: a 1 and a 3. Now, let's draw a sample of 5 values with replacement from this population. Imagine:
Pick a value and get a "1". Now put the "1" back. Now pick a second value and get a "1" again. Now put the "1" back. Pick a third value and get a "3". Put the "3" back. Now pick and 4th and 5th value in the same manner. We now have a sample of 5 values from a population of 2 values.