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When we say "the earth goes around the sun once each year" there are actually several different ways we can measure this, and thus there are several different kinds of "year" we could use.  We might take a picture of Earth and sun against the background stars and measure how long it takes for the Earth to line up with the same stars. 

Or, we could look at the distance from the earth to the sun, and measure how long it takes to go from closest to farthest to closest again.  This would give us an "anomalistic year".  What we usually do, though, is measure the time between one vernal equinox and the next one - the Tropical year. Since the earth's orbit precesses a little and since the earth's axis also precesses a little, these three kinds of year don't match exactly.

What is Precession?
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The length of the three kinds of year, in days, are:

365.2564 = sidereal year

365.2596 = anomalistic year

365.24219 = tropical year

Our calendar, with leap years, matches the tropical year rather well:

1 leap year every 4 years -> 365.25

minus one every 100 years -> 365.24

plus one every 400 years -> 365.2425

minus one every 4000 years -> 365.24225

So our calendar is good to 0.00006 days per year, or 0.00006/365.24219 = 0.0000016 = 0.00016%, or 1 day in 16,667 years