The answer is "it depends on the nature of the business".

Generally speaking, CRM covers front office business processes, while ERP covers back office business processes. However, these kinds of software are often vertically integrated (i.e. targeted at specific kinds of organisations/industries), and so at times the terms are used interchangeably.

CRM is primarily focused on the sales & marketing processes. ERP is commonly is primarily focused on getting the things you need to sell ready to sell (e.g. purchasing, manufacturing, hiring/developing employees/contractors) and managing the ordering/billing/delivery aspects of the sale. Both typically overlap in capabilities around sales.

CRM and ERP typically have different perspectives. CRM is typically customer-oriented, intended to create and build/maintain relationship with customers through managing the interaction with the customer, both directly and through sales/service partners. ERP is typically product-oriented, intended to make sure the organisation and its suppliers work together efficiently and effectively (from the point of view of being ready to meet market demand).

As a result, while large organisations typically have both, smaller organisations will have a variant of one or the other as their primary system. Smaller organisations where systemising the prepation and delivery of product is the focus will use an ERP (e.g. manufacturing), while smaller organisations where the relationship is the focus (e.g. close collaboration with the customer is required to get the sale and/or deliver the right product so the customer will become a repeat customer) will use a CRM (e.g. professional services).

Getting back to vertical integration, if a particular CRM is targeted at the professional services industry, it may include personnel/project management even though that's normally an ERP function; conversely, if an ERP is targeted at FMCG distributors, it may include sales partner program management so you can manage you distribution channels even through that's a CRM function.

Hope that helps.

Would that mean that the speed of light in our universe is a function of the speed of expansion of the universe's event horizon?