CRM tracks the customer relationship. ERP runs orders, warehouse and the books. Many companies need both, connected.
These two systems are often mixed up, because both show up around the same deal.
CRM
CRM holds leads, quotes and follow-up. Its question is: who might buy, what was already discussed, and what happens next.
ERP
ERP holds confirmed orders, stock, purchasing and the books. Its question is: how much stock is left, whether the order shipped, and whether the accounts match.
When to connect them
When a quote in CRM is accepted, that order should move into ERP so warehouse and accounting can continue, instead of retyping it. If the company is still finding customers, CRM is enough. If internal operations are the problem, start with ERP.