Enabling this option on an entity would add two new fields called Process Id and Stage Id. Process Id holds the id of the process associated with that record and Stage Id stores the id of the current stage of the record in the process.
Next add Stages and Steps in the Process flow.
You can add multiple stages in the Process flow and each stage can have multiple steps. You can use the Required field to ensure that one can move to the next stage only if the required step is completed. Here is an example of an existing process flow provided for Phone to Case
Apart from adding Stages and Steps for a single entity. You can also design the process to include multiple entities like the Lead to Opportunity Sales Process. Using the +/- options next to the primary entity you can go ahead and include another entity and make sure the next steps and stages are based on the other entity selected.
Just like we can design role based forms in CRM, the Business Process flows also provide the option of designing different processes for different kinds of users. You can assign roles to the Processes using the Enable Security Roles option
You are allowed to create multiple processes per entity and therefore you do have an option to switch the process for a record mid-way. You can start using a particular process for a record and then if you would like switch the record to another process if you find that more appropriate for the record. To switch process you should have access to that process through security roles.