Porosity goes hand in hand with texture as one of the biggest indicators of what products will work best on your hair.
This is a very abbreviated explanation, but...
Low porosity hair has a very closed cuticle and needs moisture.
Medium porosity hair needs a balance of protein and moisture.
High porosity hair has a very open cuticle and is often (but not always) damaged from heat /perms /dyes. It needs protein.
On the texture side, which is the size of individual strands...
Fine hair needs protein.
Medium hair needs a mix of protein and moisture.
Coarse hair needs moisture.
If you're lucky, your texture and porosity match as far as protein /moisture needs, but some have complete opposites, like fine /low porosity or coarse /high porosity. In those cases, it's a bit of trial and error to figure out what works.
I did the porosity test described toward the end of this blog entry
. It jived with how my hair already behaved, so it was more a confirmation of what I already suspected than a huge revelatory moment. If you want to learn more about texture, this entry
has good info.