One year is 365.2425 days. Not 365, and not 365.25 either — the figure is the average length of a year in the Gregorian calendar, and no individual year is ever exactly that long.
About the units
The Earth takes about 365.2422 days to complete an orbit, which is the tropical year. Since a calendar must have whole days, the excess quarter-day has to be absorbed somehow. The Julian calendar added a leap day every four years, giving an average of 365.25 days — slightly too long, by about 11 minutes a year. Over sixteen centuries that error accumulated to ten days, which is why the Gregorian reform of 1582 both skipped ten dates and changed the rule: leap years are divisible by 4, except centuries, except centuries divisible by 400. That gives 97 leap days per 400 years and an average of 365.2425 days, accurate to about one day in 3,200 years.
The exact factor
The value 365.2425 is exact as a definition of the Gregorian mean year, though no calendar year has that length: real years are 365 or 366 days. The distinction matters in finance and engineering, where 365, 365.25 and 365.2425 all appear as conventions and give slightly different answers over long periods. A Julian year of exactly 365.25 days is still used in astronomy, where it defines the light-year. For most purposes the difference is negligible, but over a 30-year mortgage or a century of climate data it is not.
Where you meet this conversion
Interest calculations, depreciation schedules and any rate expressed per year that must be applied per day. Different financial conventions use different day counts — actual/365, actual/360 and 30/360 are all in common use — and the choice changes the payment. Age calculation, subscription billing and warranty periods all involve this conversion. In science, annual rates of change in climate, population and radioactive decay are converted to daily or per-second figures using one of these year lengths, and papers state which.