There really is no simple explanation. However, you can generally identify a yukata versus any other kind of kimono by a few things:
1) Season: If it's not summer, it's probably not a yukata. Yukata are very casual kimono meant to be worn in the summer or as bathrobes at places like hot springs.
2) Material: If it's made of cotton it's probably a yukata. If it's not made of cotton it's probably not a yukata.
3) The collar: A regular kimono has at least two collars. One sits up close to the neck, while the other is a bit lower in the front so the first collar can be seen clearly. Basically what you're looking at is the juba collar showing underneath the kimono collar. You don't wear a juban under a yukata, though, so there should only be one collar (some people attached decorative ruffles to yukata collars for fun, but it's a very different look that the appearance of a juban collar under a kimono collar).
4) Footwear: You never ever ever go without socks with a kimono...except when you're wearing yukata. If the person you're looking at isn't wearing socks, the chances are much higher that you're looking at a yukata than a regular kimono. Some people might make exceptions for really fun patterned socks, though, so this isn't always the best way to tell.
5) Really crazy decorative stuff, like heko obi (soft, scarf-like wraps as opposed to the stiffer, flat obi), false obi jime, or an obi that has been twisted or folded in the front to show off the color on the reverse side. Heko obis might be used on their own, which is something you would never see with other kinds of kimono except possibly on small children. Some people also use them in conjunction with a regular obi to look more decorative. If someone is wearing an obi jime, look to see if it seems to be holding any part of the obi bow in place. On a regular kimono the obi jime is both decorative and useful (it helps hold the bow together), but on a yukata it's just decorative. If you see an obi that's twisted or folded in the front, you're almost definitely looking at a yukata, because you wouldn't do that kind of thing at all with a regular kimono.
I collect kimono (all kinds) and study proper kimono wearing.