Partners

Your partner is the person you cohabitate with.  It's the person you sleep with, own/rent a house with, raise kids or maybe just pets with, share your life with.  That person is your almost-spouse and is socially accepted as your spouse-equivalent.  Anytime one would invite someone's spouse, one would invite his/her partner.  When a cohabitating couple stays as overnight guests, all but uptight hosts allow them to share a room.
The only people I hear use the word "partner" on a regular basis are gay or lesbian.  Initially, I think the usage was born out of a desire not to unsettle the still homophobic by drawing attention to the sex of your partner.  But mostly it got traction because it was a way to mark that it was a committed relationship, to confer more dignity than the words "boyfriend" or "girlfriend" have. 
Straight people have partners, too, of course.  But since marriage is always a possibility, society was somewhat slow in conferring legitimacy on unmarried straight couples who live together.  Lately, those of us in such relationships are gaining rights without having to accept the status of common law marriage.  In no small part, it's because cohabitation is on the rise and couples have agitated for those rights and social acceptance.  But most of the credit probably should go to those who agitated for gay rights.
Gays and lesbians can't marry, so an alternative social and legal structure has built a piecemeal and highly variable set of rights and social recognitions that go under the term "partner".  Depending on where you live and where you work, partners can put each other on their insurance, can borrow money together, buy houses together, adopt children together, make each other beneficiaries on life insurance, and all sorts of other rights, large and small.  Socially, of course, a partner is recognized as a spouse. 
And whenever a right opens up for gay partners, it usually opens up for straight partners.  (Except, of course, for the ones that they already had due to heterosexual bias.)  I still refer to my boyfriend as my boyfriend, but on all legal documents, he's my partner.  It does cause one to wonder if the conservatives who are against gay marriage realize that everything they've done to squash gay marriage has created more need for alternatives to marriage rights and therefore has made it easier for straight couples to refrain from marrying. 
So, I watch this struggle for gay marriage with some self-interest.  When gay marriage becomes legal, will that result in the downscaling of rights for unmarried partners?  I sincerely doubt it.  That can of worms has been opened and far too many people are invested in this particular alternative to marriage for those rights to just be snatched away.  But I'm sure that once gay marriage is widely legal, the pressure to marry on both straight and gay couples will increase.  One way or another, it'll be interesting to see what happens.