What types of organisms are considered K selected?
K selected organisms produce few offspring but invest a lot of energy in them, and examples would include whales, primates (which includes humans), elephants, and some birds, like arctic terns.
This is opposed to r selected species, which produce a lot of offspring but tend to invest relatively little energy in them. Most fish and rodents would be considered r selected. You can sort of think of it as quality (K selected) vs quantity (r selected). That's a simplification of course, but it helps.
Neither K or r selection is necessarily better; both have advantages and disadvantages.
Keep in mind though that a lot of organisms lie between these two and that r/K selection theory has fallen a bit out of favor over the last few decades. Still though, it is helpful in understanding organisms' life histories.
For more info, check out this helpful video:
Bozeman Bio r/K selection