Everyone has a different experience with OCD...mine has mostly been moral/religious OCD, and I also had the classic fear of contamination (with hand-washing compulsion) as a child. Living with it can be devastating, as an obsessive person's brain makes them doubt what is actually true. Basically, we focus on the doubt, on the 1% chance that something is wrong instead of the 99% chance that it is right. For example, during my fear of contamination I would be in agony about whether or not something (say, the bathroom sink handle) was dirty. I would wipe it down, stare at it, etc., etc., afraid that it was still "dirty," feeling like there must be some miniscule piece of dirt somewhere. To a "normal" person, of course, there is the logical recognition that a possible tiny amount of dirt is not a big deal. But an obsessive person focuses on that one piece of dirt, on the small chance that it is dirty. It feels like the worst thing in the world, and they have to be 100% sure that it's totally clean. Usually OCD focuses on specific fears/themes, and as one gets solved it moves to another. It usually latches onto something it thinks will bother the sufferer intensely - so someone with a germ/contamination obsession might be terrified that they'll get themselves or someone else sick, which makes it seem like the worst thing in the world if there's even a chance that their hands or some other object is dirty. Common obsessive themes are fear of lying/cheating, fear of making an unwanted vow (e.g. to give up something you love), fear of praying to devil, fear of committing a violent act, etc. An obsessive person's brain is not strong enough to grapple logically with these themes - that is to say, they do know the truth, but it's overshadowed by a cloud of doubt, and they go round and round with those doubts and overanalyze things to death. For example, with the fear of making an unwanted vow, they will think, "Well, I think I might have just told God that I would give up reading forever." (They actually haven't they're just afraid that they have.) They're not sure, but there's that chance, so then they become terrified that they SHOULD give up reading forever, and fall into a cycle of trying to figure out whether or not they did make a vow. Having OCD is basically like having a SUPER sensitive mind/conscience, and an ability to way overanalyze things. The physical explanation is that obsessive people lack the chemical serotonin in their brains, so they don't process some thoughts as smoothly as non-obsessive people would. Where a non-obsessive person would say, "Oh yeah, that's a weird thought," and move on, an obsessive person gets stuck on it and starts worrying that it's true.
As for how OCD affects relationships - it depends, I guess. My parents have been amazingly patient and supportive, but I know they got frustrated sometimes with me when I kept coming to them with the same questions over and over. Because people with OCD are never sure about their worry, and always come up with a new angle from which to doubt it, a common compulsion (way they try to feel better) is by asking questions about it - over and over and over. "Do you think I made a vow? What if I thought this? Well then, what if I thought this? What if I said/thought this particular word? What if I felt this way when I think my mind said the vow? Does that make it a real vow?" Etc. I think it's frustrating sometimes for non-obsessive people to deal with obsessive people, because of course they see the logic in the matter and to them it's so easily solved. "It's not a big deal! Just get over it!" But to the obsessive person, it feels like a matter of life and death. Thankfully, there are ways to break the obsessive cycle and to think and act almost normally. Some people take meds and do ERP/CBT therapy, and these therapeutic techniques are usually the best way to break the cycle. Basically, it's acting against the obsessive fear and refusing to give in to whatever it's telling the person to do. So, refusing to wash hands excessively, refusing to keep asking for reassurance, etc. It's excruciatingly hard for a person with OCD to do this because they're so focused on the doubts and fears that their thinking is skewed and they feel like they can't judge normally (e.g. "How do I know I've washed my hands enough?") but if they keep acting against the fear and attempting to do only what's reasonable, their way of thinking adjusts and they are able to see clearly again. It's like they know the truth deep down inside, but they let a pile of doubt and worry bury it. Hope this helps!