A CRM, by definition is the system that helps you manage relationships with your customers (Customer Relationship Management). In typical scenarios of the past decade, this information was spread out between managers, sales people, secretaries, etc. CRM systems help managing that information.
Do I Need a CRM System?
It depends on the type of business, but in most cases the short answer is Yes. It is always a good idea to keep all that client interactions data in one place. Your employees can save a ton of time by just having all the data needed in front of them. You can analyze the relationship data and gain useful leads for further growth.
Can I Use an ERP System With CRM ?
Yes. Most modern ERP systems provide integration or even have the CRM features integrated. The benefit is that you can cross reference and analyze data for pre-sales, sales, payments, etc. to obtain a much more diverse picture of the relationship with a given client.