Understanding SumProduct?

I have just started using sumproduct and was wondering if you could help me understand the following

=SUMPRODUCT(--(A2:A12=B2),(D2:D12))
and
=SUMPRODUCT((A2:A12=B2)*(D2:D12))
are these the same function - I get the same results - just trying to figure out what the -- means in all of this.

I think "*" means add
If I did the following
=SUMPRODUCT(((A2:A12=B2)+(A2:A12=B3)+(A2:A12=B4)),(D2:D12))
then the "+" means or

New to this formula and normally use arrays but this may be better in some situations
I need to understand the the above as I created a sumproduct a few weeks ago and can not remember what it does
=SUMPRODUCT((kpi_sections=$M7)*(INDEX(KPI_ragy_data,0,MATCH($J$3,periods,0))="Amber"))


Any guidance on sumproduct apprecaited.

Thanks - great link

Try xlDynamic for a comprehensive explanation.

Cheers,