It is the difference between talking to yourself and talking to someone else.
Internal communication within a company (or department) is communication that does not leave the company (or department). It can be less formal, and can make use of jargon or "common knowledge" within the company (or department). (In the US, internal communication tends to be "discoverable" in legal cases, so may have certain restrictions imposed on it by company policy for that reason.)
External communication is communication that leaves the company (or department). It may be with suppliers, clients, partners, or members of the public, press, or financial community. Such communication must be respectful, "business-like", maintain or enhance the image the company wants to project, and may create or have to meet certain legal obligations. These restrictions tend to make it more formal in tone, and more predictable in form and/or content than internal communication might be.
Internal communication within a company (or department) is communication that does not leave the company (or department). It can be less formal, and can make use of jargon or "common knowledge" within the company (or department). (In the US, internal communication tends to be "discoverable" in legal cases, so may have certain restrictions imposed on it by company policy for that reason.)
External communication is communication that leaves the company (or department). It may be with suppliers, clients, partners, or members of the public, press, or financial community. Such communication must be respectful, "business-like", maintain or enhance the image the company wants to project, and may create or have to meet certain legal obligations. These restrictions tend to make it more formal in tone, and more predictable in form and/or content than internal communication might be.