Obviously, the following is a personal experience and isn't scientifically useful:
I have been aboard an "ET ship". When I saw it coming over the mountains in Vermont between Bennington and Rutland, I pulled over my car and in a flash it was over my head. I looked up into a very bright light and the next moment I was aboard the ship looking out at my body below me, frozen in time. Clearly, I was out of body and time was moving much much faster than on earth below. I then saw the beings' reflections in the glass. They were all lined up on the opposite side of a table (oak conference table - I'm guessing it looked like our type of table to make me feel more comfortable). I was shown the ship, talked to, and then let go. I don't remember the whole conversation, but I remember that some of it was pretty upsetting info about how we are destroying the earth, etc, and a few demonstrations of our highest potential alternate futures where I was actually experiencing it, like virtual reality and felt all the emotions of everyone on the world - pretty intense. However beyond that they made me feel relatively comfortable and I wasn't scared. The ship itself was sparse and boring. What caused them to seek me out? Not sure. I'm telepathic (better at sending than receiving, just like I'm better at talking than listening

) and had broadcasted some telepathic messages at one point (broadcasting is a bad idea unless you want a ton of beings to start communicating with you for a while, which is very distracting). Around that time they had connected to me. Perhaps that's why they chose to come visit me.
Anyway, a ship comes by (different ship than the first one) whenever I'm in a more rural area and usually just hangs around for a while. If I travel a long distance, it usually takes a couple days for them to hone in. Now that I'm in one of the metro areas in the country I don't see them around except when I leave the city. The pattern gives me the idea at least these avoid cities for some reason.
Although that first time the ship (different ship) looked more solid. This other ship is almost invisible and bends the light behind it like glass. Therefore, you can see it by walking around where I think its edges are and seeing subtle differences in how it refracts the light from trees, stars, etc. I haven't tried to touch it because it has always been at least 20 feet overhead. I can sometimes hear a humming and see energy zapping around where it is if I am not distracted by other things. One time I saw it start to move off into the sky and that gave an even more awe-inspiring effect than when I just walked around. The trees looked like they were bending as it passed upwards.
One time they walked over to me in my father's backyard (years ago, home from college). The "ETs" were only partially opaque (like they were made of only semi-solid energy) and walking ON TOP of the grass (they were so light, they only pushed it down a tiny bit). it wasn't opaque like see-through skin - I mean like if you took a video and then turned down the opacity to 75% and could see what's behind it. I could move my hand through them but their bodies stung like a continuous electric shock.
Anyway, this all leads me to believe that they are not physical like us. Because I went out of body to go on their ship, and because of their partial opacity in the backyard of my father's house, but because the other ship was visible, I'm guessing that they can take a more solid form when they want to but that their natural form is more energy-like. In fact, based on what I've seen, I'm not entirely sure what's different between them and what people call spirits.