Each Minecraft account has it's own skin, the skin can be changed without changing Minecraft username. By default you will get skin that belongs to Alex or Steve.
You can update your Minecraft skin on official Minecraft page and it should show up on your Minecraft shortly after, keep in mind you might have to restart your Minecraft client to see updated skin. Minecraft skins can be downloaded as PNG files, you can in fact download skins of specific Minecraft profile on our page.
When changing your Minecraft skins you should be careful tho, you can get banned for using inappropriate or explicit skin and Minecraft servers will most certainly ban you for that.
If you want to see Minecraft skin of specific player just enter his or hers username or UUID in search and check out profile page for that Minecraft name. You can then also download that skin or apply it to your or Minecraft profile.