CRM is your system of record and the central hub of all customer interactions - marketing, sales and support.  It provides a repository for all of your leads and contacts.  It also allows you to track where leads are in your funnel, and once those leads convert to opportunities, CRM lets you define sales processes in which deals progress through stages from qualification to closed.  It provides reporting and visibility across the organization, giving managers and reps insight into things including pipeline, activity and customer issues.  CRM also helps reps manage their day-to-day activities, reducing the problems of dropped balls and forgotten follow up.