CRM Leads are Connections that have provided their contact information to you and have the potential to be converted into CRM Contacts. For example, you participate in a conference and collect information from people visiting your booth. These visitors would be considered Leads. Your sales team members will typically call or email these Leads and see if they are interested in buying your services or products.
CRM Contacts are those Leads that you have followed up on by establishing a professional relationship, with whom you now have an ongoing interaction. For instance, one of your conference visitors is interested in your products, and you talk with them once a week as you try to close a deal, they'd be considered a Contact.