A is for ANCESTOR. Ancestors are those people with whom we have a direct blood relationship. This includes our parents, our grandparents, our great grandparents (GGrandparents), great great grandparents (GGGrandparents) etc. If you do the math, this means that researching back ten generations (about 250 years) will show you have 1024 ancestors! Before you panic, temper this fact with the awareness that very few people are ever able to research all their ancestors this far back. Envision this number and you can see how descriptive the term family tree really is.
D is for DESCENDANT. Descendants are those individuals who have a direct blood relationship with those born after them: children, grandchildren, great grandchildren, etc. Looking again at ten generations in a family, an individual living back in the late 1700s could have hundreds to thousands of descendants, all depending upon each individual's fertility and longevity. Like me, you may find one relative in the 1800s who had 24 children and others who died without issue (childless) so descendancy charts won't have a standard shape like an ancestor chart does.
R is for RELATIVE. Relatives are our "kin", to use a good old Southern term. Besides our ancestors and descendants, it is also our nieces, great nephews, sisters-in-law, step-brothers, grand uncles, and that fourth cousin twice removed on our father's side. These people won't all show up on a family tree, but they are still a part of our family and may provide more interesting stories to share.
Check out the extensive Genealogy Glossary on Genealogy.com. This way when you read of a great grandmother being referred to as a relict, you'll know it wasn't saying she was really old, just that she was a widow.