Remember when I said that the sampling distribution of sample means is approximately normal for a large enough sample size? This caveat means that for smaller sample sizes (usually considered to be below 30), the sampling distribution of the sample means is not well approximated by a normal distribution. It is, however, well approximated by another distribution: the t-distribution.
Note
A bit of history... The t-distribution is also known as the Student's t-distribution. It gets its name from the 1908 paper that introduces it, by William Sealy Gosset writing under the pen name Student. Gosset worked as a statistician at the Guinness Brewery and used the t-distribution and the related t-test to study small samples of the quality of the beer's raw constituents. He is thought to have used a pen name at the request of Guinness so that competitors wouldn't know that they were using the t statistic to their advantage.
The t-distribution has two parameters, the mean and the degrees...