Astronomically speaking, spring begins today, after 12:29 in the afternoon. Vernal equinox literally means “equal night,” meaning that the night and day on the equinox are nearly the same length – about 12 hours each.
Earth, which is tilted at an angle of around 23.4 degrees, orbits the sun in about 365 days. As Earth makes its yearly orbit, one hemisphere faces the sun more than the other – the side that has summer.
The equinoxes mark the exact moment twice a year (autumn equinox in September marks the beginning of fall) when the Earth’s axis is not tilted toward or away from the Sun at all. But the axial tilt of around 23.4 degrees, remains the same.
After the vernal equinox, the Northern Hemisphere starts to tilt toward the sun, which means longer and sunnier days.
Sources: https://www.timeanddate.com/astronomy/equinox-not-equal.html, http://wonderopolis.org/wonder/what-is-the-vernal-equinox/, The University Almanac