Thanks to TCGirl for finding this long video of a seminar, which gives a nice and clear introduction to the world of mirrorless cameras, and what they basically are, and why some of us are so enthusiastic about them.
Me, I have about 4 semi-complete camera systems. Why? I swear I didn't mean to! It was because as the digital camera developed, I always wanted the most compact camera which gave me the quality I wanted (about 12 megapixels and good lenses). About ten years ago, god help me, that was the Nikon D2x. That could only be called "compact" by somebody used to shoot large format, 8x10inch negatives... It was huge. And it was noisy at only ISO 800. On a full frame camera!
Then came the Nikon D100. Same quality, much smaller camera. Relief!
Then Canon came out with the 5D. Full frame at a relatively affordable price! I went to Canon.
Then Pentax made the K-x, a super-compact SLR with excellent features and image quality ("I.Q." as some call it). I went to Pentax.
Then, sigh, the mirrorless slow-revolution started happening. It took a while for this to really bite into me, because the autofocus was slow and they did not do well in low light. But over the years, Panasonic and Olympus, the two pillars under the Micro Four Thirds format, solved those issues with a crunch, and they came out with excellent (really) and compact lenses, and for me now (and I hope for a while!) that's my home system. For portraits and 'ultra-serious' shoots, I can still go to full frame Canon, and for a bring-everywhere camera there are several excellent choices of pocket cameras, not the least the Sony RX100M2. (Heck, that one is so good that apart from speed of operation (and no exchangeable lenses) it might be one's only camera, and still make pix good enough for exhibitions. It's even very good in low light, until recently the great archilles heal of compacts.)