All companies make use of IT these days.
An IT company may be as simple as company established for developing websites, web applications, desktop applications, mobile applications (Android/iOS) to the company that works on Big Data, AI, Machine Learning , etc. (Big Data may be of different fields such as Bank,Health, Education, Industry and Governmental Organizations.)
Some companies may work on outsourcing projects.
B2B, B2C are the business models used by a company .
IT companies are not the companies that sale products or services, they are the actual business company. But the IT companies makes software , (internet) platform (eg. B2B web app or mobile app) , analysis of data of those actual companies.
Mainly There are 7 different types of IT services companies:
- Programmers company
- Web Development company
- Technical Support company
- Computer Systems Analyst company
- Networking Company
- IT Security company
- Internet marketing company
Source : quora