Pull Ups and Diapers are different and not exactly the same thing however Pull Ups are essentially diapers but the difference is that Pull Ups can be easily removed by the child without assistance where a diaper is taped on and needs help with changing it.
Diapers are not meant to be pulled up and down to use the toilet because the diaper is meant to be used for the child to pee and poop in before they become potty trained.
Pull Ups are less absorbent diapers that are not meant to hold a lot of pee but instead the Pull Ups are just absorbent enough to catch the pee that the child releases before making it to the toilet.
Diapers hold more pee because they need to and leak less while Pull Ups can easily leak if it's used as a regular diaper.