What exactly is tadlees?

I'd like to get a good/full definition of tadlees: - This means a clear definition of the meaning. - If there are different kinds/levels I'd like to know the impact of the kind of tadlees on the authenticity of a hadith. As far as I know tadlees basically means that a narrator jumps (leave out) a narrator in his narrator chain (in most cases a "classmate" or contemporary narrator who had the same teacher) to shorten it and reach a high sanad. The point is we find that some well known scholars like Sufyan bin Uyaynah سفيان ابن عيينة (see comments on https://islam.stackexchange.com/questions/30537/what-is-the-ruling-regarding-hadith-rejecters/30543#comment52787_30543) the historian ibn Ishaaq ابن اسحاق and I guess even imam Malik are said/called (accused?) to be Mudalis! From this I assume that there might be differences between tadlees cases/reasons maybe even the Mudallis himself...?