Okay, let's imagine that we are playing a game where everyone wears a special badge on their clothes to show how important they are in the game.
In the United Kingdom, their military has lots of different badges for different levels of importance. The lowest level badges are for new players and are called "privates". The people who are a little bit more important than privates are called "corporals", and they have a slightly fancier badge on their clothes. Then there are "sergeants" who have even fancier badges and are even more important.
The very important players who are in charge of lots of other players in the UK military are called "officers". Officers have even fancier badges than sergeants and come in different levels, such as "lieutenant", "captain", "major", "colonel", and "general". Think of them like the bosses in the game where they give orders to lots of other players.
Now, in the United States military, they also have badges for different levels of importance. But, they have different names for the levels. The lowest level in the US military is called a "private" too, but then they have "specialists" instead of corporals. The next level up is called a "sergeant" just like in the UK, and they also have "staff sergeants" and "sergeant first class". These are all similar to the UK sergeants, but just with different names.
In the US military, the officers also have different names than in the UK. The lowest level officers are called "second lieutenants" and are similar to UK lieutenants. Then there are "first lieutenants" and "captains". They are similar to UK captains. The higher level US officers are called "majors", "lieutenant colonels", "colonels", and "generals", which are also similar to their UK counterparts.
So, as you can see, both the UK and US military have different levels of importance (like different badges in a game), but they just have different names for them. However, they both have very important players who are in charge of lots of other players (like in a game with bosses).