The meaning of different parts in the naming of glucose
Glucose can exist in the open form or cyclic form.  



In the open form, there are only two types, D-glucose (illustrated in Fischer projection above) and L-glucose which is the mirror image of the above.  

Experimentally, D-glucose rotates plane polarize light clockwise, hence the symbol "+", and L-glucose rotates plane polarize light counterclockwise, hence the minus symbol.  For more information on the meaning of + versus - see: http://en.wikipedia.org/wiki/Dextrorotation_and_levorotation

In the cyclic form, there is an additional chiral center, so there is alpha (where 3 OH groups are on the same side of the ring) vs beta (where 2 OH groups are on each side of the ring) as ilustrated in the drawing of the question.  

Each of alpha and beta have a D and an L form, so there are 4 types in the cyclic form.  

So in summary, there are only two types in the open form, but 4 types in the cyclic form.