HIV is a virus. You can get infected with this virus and at first you have few symptoms - a 'flu like reaction, which may repeat.
The virus attacks the immune system in your body and it is in turn atacked by the immune system. A war is waged in your body. During this time you will function more or less normally.
Eventually the virus may win the war, leaving your immune system crippled, hardly functional at all. And all the minor and major diseases that were previously resisted by your immune system are now free to wreak havoc in your body. And they do. You get multiple infections and cancers.
This heavily infected state is a syndrome (or a manifestation of a disease) called Aquired Immune Deficiency Syndrome or AIDS.
AIDS is usually fatal, though people have been pulled back from the brink by ARV drugs.
Note that HIV virus itself does not kill you - the other diseases kill. But to say that HIV is thus harmless is untrue. If someone shoots you, it is massive tissue damage due to the bullet that is fatal, but the agent that caused the tissue damage - the shooter, is responsible.
HIV is the virus that causes AIDS. When you get infected it is initially HIV. Then, when your T-cell count gets below 200 then you have AIDS.