What is permission set in Salesforce?

Permission sets in salesforce.com are the combination of different settings and permission sets given to user to access records and files.
Note :- Manual sharing is available for only Organization wide default settings are private to the object.

Permission Set provide users permission to access tools and functions on the bases of collection of settings and permissions.

Permission set: It is assigned to users. We can assign permission to multiple users using Apex data loader. It is very useful when you want to assign permission to users on an ad-hoc basis without doing any changes to the user’s profile. For example, To give users access to a custom object, create a permission set, enable the required permissions for the object, and assign the permission set to the users. We never have to change profiles or create a profile for a single-use case. While users can have only one profile, they can have multiple permission sets.