I can highly recommend reading Mono's posts on this topic that LR linked to.
To add my own perspective:
Originally Posted by WhatHappened
I'm 9 months into this, and I still struggle to wrap my mind around what a secondary relationship is, exactly. I realize it will probably be different for everyone, but I'd love to hear from those more experienced how you define your secondary relationship, what your commitments and expectations and hopes are, how does the 'love' compare...or anything else.
The love doesn't compare. I can't compare my love for any two people - each relationship feels different and I can't rank them in terms of "more" or "less".
For me, a secondary is just someone that I don't live with and don't share the day-to-day life decisions with. We talk nearly every day at length, and tell each other about our days. We meet each others friends and hang out. We help each other with life.
What's the degree of commitment if it's not moving in and making babies? To me, a committed relationship doesn't mean "till death us do part", it means that we're not going to walk away at the first sign of trouble, or on a whim. If there are problems, we are going to work together to try to resolve them, to find a win-win ideally, or a compromise is a win-win isn't available. We become interdependent (and if you're not sure of that term, I can provide some references) - part of each others support structures in life. Someone that can be relied upon if things go pear-shaped. Someone to lean on or a shoulder to cry on. Someone that will be prepared to listen to us rant about how crappy a day we've had at work, or how tired we are. All this in addition to someone to do fun things with.
That's what it means to me.