Between subjects effects refer to how differences between subjects (people, teams, organizations, companies, etc.) affect how they behave or perform. Many of the differences we are interested in are like this. For example, how does web traffic received from two sets of accounts differ, where one set of accounts was exposed to advertising and the other set was not. Being exposed to advertising or not is a difference between the groups of subjects (in this case, accounts).
Similarly, we might want to test whether we receive more traffic to our website from companies depending upon the industry of the companies, their size, or how those companies are funded. Each of these are between subject effects.
ANOVA tests are capable of testing both within subjects and between subjects differences.
