At its core, the business cycle refers to the fluctuations, at a broad level, between economic expansion, and contraction, helping to define the overall state of the economy at a given time. Many indicators can be used in an attempt to time the economic cycle, including - GDP, interest rates, employment, and spending.
Typically, the business cycle is split into the four stages below. However, it is important to note that no two cycles are identical; the length of expansions has varied wildly over time, as has the length of recessions/depressions, while the amplitude of the peaks and troughs in the cycle also varies over time.
A graphical representation of this cycle can be found below, courtesy of the St Louis Fed: