Spin is a Quantum-mechanical property of particles, measuring, as you might guess, angular momentum.

Spin is measured in terms of Planck's constant of angular momentum, which is symbolized by h with a diagonal line through it. It is equivalent to Planck's constant h divided by 2*pi (the number of radians in a full circle). That is, 1.05450*10-34 J*s.

One might think that a basic quantum-mechanical measurement might always be an integer multiple of its basic unit, but spin is measured in multiples of one-half Planck's constant of angular momentum. Particles are divided into two types based upon their spins:

Bosons, particles with integer spin, and
Fermions, particles with non-integer spin (some integer plus 1/2)

All Baryons are fermions, since they are made up of three quarks, each with a spin of 1/2.

The alignment or non-alignment of the spins of any given group of three quarks results in two different types of baryon. For example, the spin-aligned version of the proton is the delta+ particle. Both particles consist of two "up" quarks and one "down" quark.

The spin-aligned baryons (with a spin of 1 1/2) have higher energies and masses, and are less stable, than their non-spin-aligned counterparts (with a spin of just 1/2).