TVtropes.org is a great site. Not only does it explain all the neat concepts from all types of fiction, it also allows you to look at your own subconscious expectations about fictional and even real-world concepts. (A word of warning, though: Clicking on any tvtropes-Link might lead to excessive reading of a cascade of linked pages, resulting in the loss of lots of time. )
When it comes to STO, the article about the Standard Sci-Fi Fleet is particularily interesting, because... most new folks who come to the game will assume the ships in STO to follow that logic - because that's how we see starships on TV all the time.
But that's a problem. According to TVTropes, a standard Sci-Fi fleet consists of
Small craft: Escape pods, fighters and shuttles (and, where available, Mecha)
Capital Ships: Battlecruisers, Battleships, Carriers, Battlestars, "Banner Ships"
"others": Torpedo boats, Science vessels, Space Guns
I think many people will, when first starting to play STO, believe that apart from the small craft, they see some kind of trinity in these categories, which they will expect to map to the trinity in the game. They will quite likely, even if subconsciously, expect
Escorts of STO to be the "Space Ships"
Cruisers of STO to be the "Capital Ships"
Science Ships of STO to be the "Other Ships"
But that is not true. In fact, STO does it more like this:
lower tier ships of STO are the "Space Ships", with a few of them instead being "other ships"
Cruisers, Escorts, Science Ships and carriers of Tier 4+ in STO are the "Capital Ships"... except maybe for the science vessels.
All of them are capital ships? Well, makes sense. After all, player ships are heroes, heroes get the shiny hero ships. Now, if everything is a capital ship, how do they differ from each other?
TV tropes (and thus, quite likely, our subconscious expectations) divides the capital ships along the following categories:
Battlecruisers: Fast and agile, big guns
Dreadnoughts: Slow and fat, sturdy, mighty guns
Carriers: Not many guns, but lots of fighter wings to launch
Battlestars: Big guns and some fighter wings to support it
Banner Ships: Not much offensive potential, but good for hosting the command party and collecting information.
And again, most people will, probably subconsciously, fit the STO ship types into those categories:
Escorts in STO as Battlecruisers.
Cruisers in STO as Dreadnoughts
Carriers in STO as Carriers
Science Ships in STO as... uh... what are they?
And, huh, what is the banner ship?
But that is incorrect. In fact, only one of the above is what the ship class does.
The "escort", "warship" or "destroyer" in STO is the Battlecruiser. Fast, agile, not as heavily armored as it could be, but maneuverable and with really big guns. Makes sense. But that is where it ends:
The cruiser in STO is not the Dreadnought. It does not have an armament that will, in effect, be as powerful as the "battlecruiser's", but the TV Tropes Dreadnought is defined as having at least the same offensive as a battlecruiser, possibly even more so. So, what is it? Well, big defensive capabilities, not so much offense... it is right there: The cruisers are the "Banner Ships". Command ships. They don't do much damage, they direct others and stay just operational in the middle of it all.
So that is the first fallacy that a new player will possibly fall for. Cruisers are not the Dreadnoughts. But then, where is the Dreadnought in STO? Simply put: It does not exist. There are no Dreadnoughts. There are only Battlecruisers and banner ships.
And, just on top of it:
Carriers in STO are not carriers in the TV Tropes sense. They have some fighter hangars, but the bulk of their offensive power still comes from their own weapons and special abilities. So what are they? That's right, the Atrox, the Vo'Quv, the Kar'fi... they are Battlestars, just like the Armitage and the Vesta (well, the latter also a battlecruiser and a science vessel). All of them have slightly different foci, but they are all Battlestars, not Carriers. Where is the Carrier proper? Same answer as for the Dreadnought: There is no Carrier.
So, what are science ships? They are not in the Capital Ship list of TVtropes, right? Right! Science Ships in STO are something sort of unique to STO, a hybrid between a battlecruisers (fast and big guns) and an "other ship", the "Science Vessel" of TV Tropes. A bit more powerful in using its weapons, and obviously looses the plot-protection-level wizardry of some fictional hero-ship science vessels in exchange for those weapons.
Ah, now that the misunderstanding is cleared up, everything is fine, right? You want to command a Dreadnought, but there is no Dreadnought... so you take the one thing that is closest to it: A Battlecruiser. Problem solved. You want to command a Carrier, but there is no carrier, just a Battlestar. So you take that one. Problem solved.
Just throw away all the years of being conditioned by fiction for certain expectations. No problem, no worries.