One parsec is 3.2615637772 light-years. Both units measure interstellar distance, but they come from completely different places: the light-year comes from how far light travels, the parsec from how telescopes actually measure.
About the units
The parsec is defined by parallax, the apparent shift in a star's position as the Earth moves from one side of its orbit to the other. A star one parsec away shows a parallax of exactly one arcsecond when observed six months apart — the name is a contraction of parallax second. That definition ties the unit directly to the observation: a measured parallax angle in arcseconds converts to a distance in parsecs by simple reciprocal, with no other arithmetic. Since 2015 the parsec has been fixed at exactly 648,000/π astronomical units, which removes the small ambiguity that came from the old trigonometric definition. The light-year, by contrast, is a communication unit — intuitive to a general audience, but with no relationship to any measurement an astronomer makes.
The exact factor
The exact value is 648,000/π astronomical units, which works out to 3.26156377716... light-years, or about 3.0857 × 10¹³ kilometres. The reverse is that one light-year is 0.3066 parsecs. Useful multiples: a kiloparsec is a thousand parsecs, used for distances within the galaxy, and a megaparsec is a million, used for distances between galaxies. The Milky Way is about 30 kiloparsecs across, and the Andromeda galaxy is roughly 780 kiloparsecs away.
Where you meet this conversion
Professional astronomy uses parsecs almost exclusively in research literature, catalogues and survey data, because it is the unit the measurements arrive in. The Gaia spacecraft, which measured parallaxes for over 10⁹ stars, publishes distances derived directly from parallax angles. Light-years appear in press releases, museum displays and popular writing, so the same object routinely has two published distances that differ by a factor of 3.26 — and mixing them is a common error in secondary sources.