First, you have to realize that OpenERP CRM is generic enough to allow you to decide what you would want leads and opportunities to be.

Typically, a lead is a "customer - salesperson - sale order" association. (And a customer can be a person or a company). This is because only one customer can be assigned to a lead or opportunity; only one salesperson can too; and the end of an opportunity life cycle is a sale order.

Now, what will make you decide to convert a lead into an opportunity is whether you would like a salesperson to dedicate time on a lead. Therefore, typically you will work in mass on the leads (mass emails, marketing campaigns, assignations,...) and individually on the opportunities (meetings, phone calls, custom proposals,...). Again, it is up to you to work otherwise if you want to and it is even possible to disable the leads concept and work exclusively with the opportunity object.