The latest findings show that calcium citrate is most absorbable in supplement form, and may be taken between OR with meals. However, calcium citrate is difficult to pack into a pill. You have to take lots of pills to get the RDA of 800 mg. This is a very expensive way to get your calcium. Calcium carbonate is the next best form, and a lot more calcium is available per pill.
Vitamin D helps assimilate calcium into our bones.
Generally, drinking fortified milk and getting some sunshine is enough for most people. Elderly folks sometimes have a problem in this area.