For me, raids in an MMO is most fun. This is why I always loved WoW, however, I wondered if theres something like raids in TESO? Like really really hard bosses which you can waste your whole sunday on even if you have a team of experts :D
This sounds like adventure zones will be exactly what you're looking for:
Craglorn will be the first adventure zone offering content for grouped players. They'll introduce the so far hardest content in the game in the form of so-called Trials, which will be encounters for groups of up to twelve players. There's also some kind of leaderboards (time based), so you can compare your own performance to that of others if you really want to. Those encounters won't last that long, but you can repeat them at any time and they'll have randomly picked enemies, so it's not like you've got the same content over and over again.
Besides that, if you're into PvP, I bet that the Alliance War would be something for you. It's a lot more dynamic than classic PvE raids and it will always offer you different challenges, different enemies, etc.
As an example, have a look at Angry Joe's PvP impressions:
You can do PvP alone or in a group - it's your choice and you'll never know what you're up against next (so if you like challenges, you can try to take on larger forces with your group and make yourself a name).
I found It really surprising that it was this hard to find some Information about raids in TESO but I finally found an article that should describe it. Unfortunately I can't tell these things from personal experience in TESO but rather from 5 years of Progress-Raiding in WoW and Research about TESO raids.
If you're asking whether or not raids like in WoW are in Teso the answer is NO.
Now let me first explain why raids like in WoW can't exist:
While the combat is simpler than in WoW, you are not restricted when it comes to class developement. You can play a Healtank/Smiter/Supporter/Offtank well basically everything you want. This offers a lot of different tactics for content while In WoW the only variations you will find are things like "Oh let's try this boss with 1 Heal respeccing for DPS" which doesn't make too much of a difference.
When it comes to something that can compete with WoW raids in terms of length there will be nothing. As Zenimax stated the length of a Trial (which is basically a 12 man "Raid") will be around 0.5 to 1.5 hours which is nothing in comparison to a Real WoW raid. Also a thing is the limitation of resurrections in Trials. Currently you can be resurrected 60 times otherwise the trial will end (Think of it like the Hardmode-Crusader Raid)
For the difficulty I'm not completely sure. The game definitely has the potential to be far more challenging than WoW but as already mentioned... Whether something is difficult or not is a really opinion based thing.
Here I also found an article about endgame content which describes everything a bit more detailed.