If you're using Salesforce in a sales capacity (read: Sales cloud) then you're probably leveraging the Lead object to manage your Lead pipeline and then you're converting those Leads into Accounts, Contacts and Opportunities so you can better work with them and manage their sales potential in your organization.
Converting Leads is both something Salesforce can do by default and also in a custom manner. Standard Lead fields convert into other standard fields on standard objects (Accounts, Contacts and Opportunities). Here is a key page that illustrates all the default Lead table mappings. For mapping custom fields on the Lead object, use the Map Lead Fields button in the Lead Custom Fields on the Lead object set-up screen to manage and define conversion actions.