When we speak of CRM Integration, this means that all aspects of the enterprise are brought together to function seamlessly as just one system, because the term integration tells us that the different parts should work together for the betterment of the whole.
Thus, CRM Integration may be used to mean the process by which seemingly disparate components of the same system will be employed to work together in a cooperative manner.
Most likely, your organization will be using one CRM application from just one manufacturer. But it is also likely that the organization has one product from one manufacturer and the other product from a different manufacturer. This is where compatibility may either work or be affected negatively by the use of different products from different sources.
For a system that uses different CRM products from different sources, compatibility will occur only if: the different products work along similar or the same lines; adhere to a specific and common protocol (maybe even an industry standard): the developers had the same development model in mind when they created them; they have similarities in programming code (and hopefully more similarities than differences); that there may be some special code used to tie together the two or more CRM products into the same application suite; and that whoever led the CRM Integration effort really knew what he or they were doing so that the enterprise functions well even after deployment and continuous use of the different systems (now made one through integration magic and talent.)