What is the difference between a One-Time and Recurring commitment?

What is the difference between a One-Time and Recurring commitment?

One-Time commitment:
This type of commitment indicates to volunteers that they can drop-in to volunteer at your organization with no recurring commitment.

Recurring commitment:
This type of commitment indicates to volunteers that they must make a commitment for a specified period of time.
You must indicate the # of hours, frequency and duration of the commitment, and the starting date.