Pipelines (formerly called "lead types") are for tracking the different kinds of leads or relationships you manage separately from each other. 

Processes are for running your day to day operations, and are part of the Operations plan.

For this example, we'll use a pipeline, but deleting a process follows the same steps.

If you accidentally created a pipeline and need to delete it, click the name of the pipeline under "Leads" in the sidebar. (For Processes, click "Processes" in the sidebar, then the name of the process.)

Then click the settings icon in the upper right-hand corner of that page. Or click Settings  > Pipeline Settings (or Process Settings for a process) in the lefthand sidebar. 

Next, click the General tab in the lefthand sidebar. Under the "DANGER" section of the page that appears, click the red, delete button.

You will now have the option to merge the leads that are in this pipeline into another pipeline so they don't get lost. Select the necessary options in the window that appears. Then click "Merge and Delete".

It can take a few moments for the pipeline to be deleted and disappear from your sidebar.