Interest is used most often to describe something you like, enjoy or think useful. You have an interest in 19th century literature, or you follow the news with interest.
Although interest is used mainly to describe anything that is, well, interesting, it also is used in specific legal contexts. If you have an interest in a company, it means you own part of it. Interest is also the word used to describe money charged or given as part of a debt. If a bank loans you $10,000, you have to pay back the $10,000 plus the interest. If the interest rate is 5 percent annually, you would owe $500.