This is a big question. Here’s how wikipedia defines it:
Customer relationship management (CRM) is a model for managing a company’s interactions with current and future customers. It involves using technology to organize, automate, and synchronize sales, marketing, customer service, and technical support.
In different contexts, it means different things… For example, a B2B company that sells enterprise software may have a long lead time to sales – with many pre-sales touch points. Whereas, an ecommerce outfit may sell something within 5 minutes after an initial interaction.