What are the role of the CAPFs ?

The MHA manages 7 central police forces : 

i. CRPF , which assists in Internal Security and Law & Order
ii. CISF , which protects vital installations ( like airports ) and PSUs
iii. NSG , Which is a special counter-terrorism force
iv. BSF 
v. ITBP }------- Border Management

vi. SSB

vii. AR

What are the key functions of Ministry of Home Affairs [ MHA ] ?

The Ministry of Home Affairs ( MHA ) is primarily responsible for : 

i. Maintenance of internal security
ii. Governance issues between the central and states
iii. Disaster Management
iv. Border Management
v. Administration of Union Territories
vi. Implementation of provisions relating to the official languages
vii. Conducting the population census every 10 years

Under the constitution  'public order' and 'police' are state list subjects.The MHA assists the state governments by providing them : 

i. Central Armed Police Forces

ii. Financial assistance for modernising state police forces , communication equipment , weaponry , mobility , training and other police infrastructure.