A sample is simply a subset of the population that will be used in your research. If your population is 10,000 people, you will only be using a percentage of those in your actual experiment. A sample can be representative, where the demographics are proportional to the demographics of the population. It can also be random, where you select 30 people (for example) without picking out specific demographics. There are a number of different ways to go about selecting a sample, but in all of them, the sample represents a section of the population.