There's always a huge problem within BDSM (or just about any subculture really) of definitions. There's not exactly a Websters Dictionary Kink Edition to refer to. A lot of words get used interchangeably by some people (like sub and slave, Domme and Mistress, etc), and for others there are very concrete differences between the terms. What's worse (I think) is that often people who have personally defined definitions of certain words are offended or angered by people who are either inexperienced or use terms loosely.
The OP's question is one I struggled with for a long time myself in coming into BDSM. I work with words for living, so they carry a lot of weight for me in their 'official' meanings. The label 'slave' horrified me because of the historical baggage that word carries. People being forced to serve against their will and given no respect as human beings. I've come to learn that within the lifestyle, slave can be a very proud and noble title for a person to have.
Likewise, I find a wonderful humiliation pleasure in being called a little slut by someone I'm with. In that context, it's an affirmation and affectionate word of myself as a sexual being. The word whore was one that always made me angry. The difference as I saw it was this: A slut someone who expresses their sexuality freely and openly for someone they love and serve. A whore is someone who will show that same part of themselves to anyone who can pay for it. One was personal and loving, the other was business and nothing more.
What it comes down to in the end though, is that terms and words within the lifestyle are very fluid and personal things. What's most important (again, I think), is communication with your partner.. making sure you both have a similar view on terms and their meanings.
