You can play all the DLC at any level, you will be scaled to the zone so you can partake in the story/quest/bosses even at low level.
Orsinium is a PvE expansion with a new zone, which feels much more like Skyrim, some daily quests to kill world bosses, two new public dungeons, one single player instanced PvE arena.
Thieves guild adds a small new zone focused on being a member of the thieves guild, opens up a new skill line based on thieving activities, a new trial which scales to your level.
Imperial City is a PvE/PvP zone where you'll be flagged for PvP any time you're inside it. It comes with two new 4 player dungeons (these are pure PvE), multiple strong sets, two new pets (earned via special achievements of bosses), a new skin (boss drop).
I'm sure I have missed some stuff but others will be able to flesh it out, hopefully this helps.