Word Distinction: Habits and Practices

A habit usually means something which is done regularly by a single person:
He has an annoying habit of biting his nails.

A custom usually means something which has been done for a long time by a whole society:
The custom of giving presents at Christmas.

Practice can mean custom, but often with a derogatory meaning:
The practice of eating one’s enemies

It can also mean the usual way of doing things in business, law, etc:
The normal practice in this company is to send the bill as soon as the job is done.

The conventions of a society are its generally accepted standards of behaviour:

As a matter of convention, people attending funerals wear dark clothes.