An ISP is an Internet Service Provider. This is someone who provides you with an Internet service, such as being able to connect to the Internet using a broadband, leased line, satellite or dial-up connection.
ISP is a broad term so often includes MSPs (Managed Service Providers) and ESPs (Email Service Providers) as well as Internet domain hosting and web hosting companies etc. In this manual we generally use ISP to cover all these cases.
In many situations the connectivity provider also provides email services and web hosting so is the same company, but you may use one company for your Internet connection and a different company for your Internet domain registration and hosting and yet another company for your email services. There is no need at all for them to be related.