Closed: Sold Opportunities. Closed are opportunities you've already sold and are at 100% probability.
Commit: Very high probability to close. Commit are opportunities that have given you some kind of commitment to buy and are at very high probability, like 90%.
Best Case: Fairly good probability to close. Best Case opportunities have a fairly high probability of closing, like 70%.
Pipeline: Moderate probability to close. Pipeline are opportunities with lower probabilities of closing, but they still have a high enough probability to even be listed, such as 25%.
If your organization uses Forecasting, it will probably have its own criteria for which types of opportunities go into each forecast category.