pH is a measure of much H+/OH- there is in solution (how acidic/basic a solution is, if you will). It varies on concentration.
pKa on the other hand is a measure of how acidic a molecule is (the ability to lose H+) and won't vary with concentration of a solution it's in.
If you look up the pKa values for various types of aqueous acids (or what we call acids) then strong acids like HCl and H2SO4 will have negative values. Weak acids like ethanoic acid will be around 5, phenol about 10.
If you want to think of a chemical that is neutral in water - say propanone - it's pKa is 25. A very high pKa (such as hexane pKa ~50) does not mean it's basic however, just that it's not at all acidic.