A year is the duration that the Earth needs to complete its orbit around the Sun. This duration is about 365¼(365 days and 6 hours) days long.
In the most used calendar, called Gregorian calendar, a year is a period of 365 days starting from the first of January. To catch up with the missing ¼ days, one day (the 29 February)
is added every 4 years. A year counting 366 days is called a leap year.
The year is commonly divided into 12 months and 52 weeks (although some years, such as 2015, are considered having 53 weeks according to the international standards).
Some calendars, such as the Chinese calendar, are based on a combination of Moon and Sun cycles. The length of the years may then vary a lot.
A year may also refer to the amount of time somebody has been alive, an example, Johnny is 11 years old.