Leads can be recorded and tracked against your marketing campaigns and eventually converted to opportunities. Accessing a single lead page will show you all associated information including calls, notes, emails, meetings held...basically every interaction on one page. This is the page that will pop-up if your lead calls, giving you all the information you need to progress with your prospect.
Leads can be entered manually, through your website or any other internal or external system. Permission based roles can assign leads to any individual within your organisation.