SureAI

At the beginning of the game it was explained what the asterisks mean in the list of quests, but I forgot it.
I've not founded help in the FAQs.
Can someone please help me?

They indicate the difficulty of a quest.

My question is for each mission difficulty is there a Lvl requirement as in (**) = 30 etc?

I don't think there is any specific level requirement for each quest, the asterisks indicate generally how long it might take and how difficult the enemies might be, if there are any. I've had some (***) quests I thought were rather easy but involved a lot of running around to different places, and one that I thought should have been (*****)! There are some quest chains that might start with (*) and gradually work up to (***) or (****) as you make progress.
For example:

[+]

There is the quest to gather 5 eggs for the half-starling guy near the watchtower of Ark, pretty simple, I think it was (*). But when you complete it, he gives you 3 more quests that are (**) & (***) as I recall, because you have to travel far and the nests are in areas where you encounter stronger wildlife.

For example: Examine the cave in King's Bay (**). Different quests have different amount of ***, I found that places that are harder to reach have more stars. So i wanted to ask, whether my deduction that stars show the level of quest difficulty is correct

AFAIK:

Very easy*
Easy**
Normal***
Hard****

You should also take in consideration that enemies don't match their levels with yours, so things may get hard if you go to a high level area or if you main skill is Psionic.