What is the difference between a submissive and a slave? Is there one, really? Can the terms be used interchangably?
The simplest answer between the difference between a submissive and a slave is summed up in one word: Negotiation.
Simply put, the submissive has the right to negotiate new rules for herself, or to alter rules within her relationship after she has enetered into the relationship.
In a more general sense of the word, though, a submissive is not typically interested in a TPE type relationship. She will enter into a 24/7 domestic relationship with a dominant and will be more interested in the aspects of serving, both/either domestically and/or sexually. She is not so much interested in having her life micromanaged, nor is she desirous of corporal punishment, heavy scenes of bondage or strict discipline. She is, however, comfortable with some rules, discipline and in knowing she is in a position of perceived subservience to her dominant.
A slave, to flip the coin to the other side, is interested in being micromanaged. She wants her clothes and food chosen for her. She want to be told how to act when in public. She wants to give up her control over things and her right to negotiate. In short, she gives herself totally to her relationship and to her master.
And these terms do not even describe other forms of 'submissives' in other types of BDSM and kink. There are quite a lot of types of people that can be grouped into the broader 'submissive' category. Included in this category are bottoms, painsluts, cumsluts, piss whores, toilet slaves, masochists and a broad variety of other types that I probably don't even know about. Still, in most common terms, the word submissive covers them all.
The biggest difference, though, between what is usually termed a "true" submissive and other types of fetish/kink submissive types is that a submissive gets her first and foremost satisfaction from the act of submitting. A submissive wants to serve and please her dominant before any other consideration is taken, but not to the point of giving up her safety or putting her submission before her children, for instance.
A slave also wants to serve, but a slave's primary motivations are usually a little different from a submissive's. Certainly, she still wants to please and satify her master, but she also wants to be owned completely and have most of her decisions taken from her on a day-to-day type basis.
Now, there are plenty of people that think that submissives are just playing at being slaves and there are plenty of people that think slaves are just people that don't want to take responsibility for themselves and want all their decisions made for them. The fact is that submissives and slaves are equal, both in commitment and in servitude. They just choose to show their service and commitment in different ways.