Many international students are visited by their family or friends from home while they are in the UK. The normal visa that people would use for this is the Standard Visitor visa, commonly called a tourist visa.
Depending on where your visitors come from, they might be able to obtain this visa on entry to the UK at immigration rather than applying for it in advance. They can use the UKVI checking service to see if they need to apply for a visa in advance.
If your family/friends are able to obtain entry to the UK at the airport without applying for a visa in advance, they should still ensure they have documentation with them in their hand luggage to show the immigration officer (guidance on this is provided via the UKVI checking service).
Important to know: 
If your family/friends need to apply for a Standard Visitor visa from outside the UK, please refer to the UKVI website.