There are three common cycles for reading the Torah:
In the annual cycle, the entire Torah (Genesis through Deuteronomy) is read weekly over the course of one year. This is most widely used Torah reading cycle and is used in synagogues throughout the world.
In the triennial cycle, the entire Torah is read weekly over the course of three years. The readings are aligned to the annual cycle (both the annual and triennial readings are on the same Parashah each week) but smaller portions are read in the triennial cycle. Thanks go to the Jewish Theological Seminary for the chart!
In the linear triennial cycle, the entire Torah is read weekly over the course of three years but in a sequential order that does not align to the annual readings (the annual and linear triennial readings are only the same one week every three years).