Back from the 1810's to the 1870's or so, most people considered Ceres, Vesta, and things like that to be planets- they were bodies that wandered around the solar system, that meant they were planets. When the numbers started to get into the 20's, everyone decided to create a new category, "asteroid" (Greek for 'star-like') and put all of the smaller things in that. So when Pluto was discovered in 1930 it was slotted right into the planet category. Pluto was discovered mostly by accident, because Clyde Tombaugh was amazing at working the blink comparator, and finding the one dot that moved in between the two pictures of the night sky a few days apart.
However, by the 1990's and 2000's you had computers and digital cameras, which are even better than Clyde at finding things that move, and quickly the number of planets started to go up- and it was clear that once we had thoroughly mapped the ~~Oort Cloud~~ (meant Kuiper Belt, see below) etc. we would have dozens of planets. And so once again astronomers decided to create a new category, just like they had with asteroids a century earlier. This time they drew the line in such a way that Ceres got moved from asteroid to dwarf planet- it has enough mass to be roughly round, so after over a century of being an asteroid it became a dwarf planet.
This is how things always work in science: we discover something, then we discover more of them, and re-categorize everything based on the new discoveries. It's just more noticeable with Pluto because reciting the planets is done by every schoolkid in a way that they don't for subatomic particles or for species of voles or whatever.
