A bank is an institution where customers can save or borrow money. Banks also invest money to build up their reserve of money. What they do is regulated by laws. Those laws differ in different countries. The people who run a bank are called bankers. Certain banks deal directly with the public and they are the only ones which an ordinary person will deal with. Other banks deal with investments and international currency trading.
Customers' money may be placed in the bank for safe keeping. Banks may give loans to customers under an agreement to pay the bank back at a later time, with interest. An example is getting a mortgage to buy a house or apartment. Banks also can use the money they have from deposit accounts to invest in businesses in order to make more money.
In most countries the rules for banks are made by the government acting through laws. A central bank (such as the Bank of England) adjusts how much money is issued at a particular time. This is a factor in the economy of a country, and the government takes the big decisions. These "banks of issue" take in, and issue out, coins and banknotes.