The YDNA is the direct paternal line test. It can tell you information about your direct paternal line, or your father’s father’s father’s father, etc. Only males can take the test, so if you are a female, you will need to use your powers of persuasion to get a brother, father, uncle, or cousin to test for you.
To further simply your life, you can only order the full YDNA test from one company: Family Tree DNA. They have three testing levels starting with 37 markers and moving up to 111 markers. In general, you can start with the 37 marker test as it is usually enough to tell you if you are, or are not related to someone. The 67 and 111 tests can then help to further refine your relationship. You can always upgrade later without resubmitting a sample (just pay them more money!).
The YDNA Test Give You Two Types of Results:
Y Chromsome Haplogroup
You get haplogroup information which describes deep ancestry. When I say deep, I mean, deep. We are talking up to 10,000 years ago.
You also see "matches". These are other people who have tested their Y chromosome that are exact matches or very similar to yours. These matches likely share a common paternal ancestor with you.