How do you track customers?

How do you track customers?
There are several ways to track your customers:

Invoicing tools.
File system.
Online calendars.
Filters and labels.
Customer relationship management (CRM)

There are several ways to track your customers:
  1. Invoicing tools.
  2. File system.
  3. Online calendars.
  4. Filters and labels.
  5. Customer relationship management (CRM)