A Payday loan is a short-term loan. Originally, they were designed to tide people over until payday.
The money gets paid directly into your bank account, following which you repay the money in full (with interest and charges). The repayment will usually be due at the end of the month.
However, increasingly, you can borrow for longer periods, which will usually be three months, but longer loans are available. You repay this type of loan in instalments.
All these loans share common ground; they are high cost and short-term, often for small amounts.
You will usually have until payday to pay back your loan (plus interest), although some payday lenders let you choose the repayment period.
A key point to note is that payday loans are expensive and could make your situation worse, especially if you can't afford to pay them back on time. You need to think very carefully before choosing one.