G0 is a special phase of the cell cycle, which consists of G1 phase, S phase where the cell’s DNA is duplicated, G2 phase, and finally mitosis where the cell divides to make two daughter cells. During G0 phase cells withdraw from the cell cycle and are dormant and do not grow or divide. This is a way for multicellular organisms to control cell proliferation. The time cells spend in G0 and the specific signals needed to move the dormant cell back into the cell cycle vary greatly depending on the type of cell. Cells can remain in this phase for days, weeks, or even years. Most of the cells in multicellular organisms, including humans, are currently in this phase. For example, muscle and nerve cells are permanently in a state of G0. Liver cells also remain in this phase unless they are stimulated to grow after an injury.
At a certain point in G1 phase the cell monitors internal and external environments to determine if it should go through the entire cell cycle and divide. If the cell senses that there are not enough nutrients such as amino acids or growth factors for division then it often enters into G0 phase. During this time normal cellular activities are drastically reduced. For example, protein synthesis is inhibited by 50 to 80% and many proteins are degraded. Enzymatic activity and RNA synthesis are also severely inhibited. Cells in G0 can quickly reenter G1 and progress through the cell cycle if they receive signals that nutrients are available for cell division.
Cells that are grown in tissue culture can be arrested in G0 phase by removing the serum, which contains growth factors, from their media. The full arrest can take anywhere from 1 to 48 hours, depending on the cell type. The cells can then be activated to reenter G1 phase and continue through the cycle by adding back the growth factors.
Source: Albert's Molecular Biology of the Cell, 3rd edition.