They're quite unique... You can fly everything - from a small scout ship, over a carrier, up to a freakin' huge space station transport ship! But flying is only a very small part of X. After a while you'll start to scan asteroids for minerals to build mines on the rich ones. You'll build huge space stations with enormous solar panels to produce energy cells. You'll build all kinds of space factories to build all kinds of stuff, like computer chips, missiles, sattelites, etc. Every station needs different ressources (energy, food, ores, etc.) to produce their goods, so you'll have to build a trading fleet to ship goods from one of your factories to another. But some sectors are very hostile, so you'll have to start to build a military fleet to protect your trading routes. And then there are some very good ships that you can't buy, so you'll have to train some marines and board these ships (needs lots of special equipment and training)! Later, when you have your HQ, you can disassemble these ships to get a blueprint.

X is huge. You can play it as a pirate (steal goods and ships), as a mercenary (get a good fighter and later a capital ship), a merchant, or build a huge enterprise. Start a war against some species and eliminate them if you want. The universe is yours. It's a huge sandbox.