What are Group A Services and Group B services in civil service exams?

Civil Services Exam Syllabus

Civil Services include All India Services, Group A Services and Group B services.
All India services
1) Indian Administrative Service(IAS)
2) Indian Foreign service(IFS)
3) Indian Police Service(IPS)

Group A Services
1) Indian Postal Service
2) Indian Civil Accounts Service
3) Indian Railway Traffic Service
4) Indian Audit and Accounts Service
5) Indian Customs and Central Excise Service
6) Indian Defence Accounts Service
7) Indian Revenue Service
8) Indian Ordnance Factories Service
9) Indian Railway Accounts Service
10) Indian Railway Personnel Service
11) Posts of Assistant Security Officer in Railway Protection Force
12) Indian Defence Estates Service
13) Indian Information Service
14) Indian P & T Accounts & Finance Service

Group B Services
1) Railway Board Secretariat Service
2) Armed Forces Headquarters Civil Service
3) Customs Appraisers' Service
4) Delhi, Andaman & Nicobar Islands, Lakshadweep, Daman & Diu and Dadra & Nagar Haveli Civil Service and Police Service
5) Pondicherry Civil Service