What is the difference between G1 and S phase in the cell cycle?

Both Gap 1 (G1) and Synthesis (S) phase are part of interphase. However, there are differences in the G1 and S phase in the cell cycle.

G1 Phase

G1 phase is the first of four phases of the cell cycle that takes place in eukaryotic cell division. During G1, the cell reviews the cellular environment and the cell size to ensure that the conditions are appropriate to support DNA replication.  G1 phase is particularly important in the cell cycle because it determines whether a cell commits to division or to leaving the cell cycle.

S Phase

S phase is the phase of the cell cycle when DNA packaged into chromosomes is replicated. During S phase a number of events additional to chromosome replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Since accurate duplication is critical to successful cell division, the processes that occur during S-phase are tightly regulated and widely conserved.