There are two parts to ejaculation referred to as Ma'thi and Ma'di.
The sperm is Ma'di which is not najis therefore only requires Wudu unless there was sexual contact after which Ghusl would be required.
The liquid that comes before the sperm is Ma'thi and is classed as najis and also requires Wudu only.
How ever in combination to a complete wet dream one would be required to Ghusl.
The difference in the liquids is one Ma'thi is snotty and sticky like and Ma'di is whitish thick and yeast like smell.
What is mentioned above is a consensus of the four Mathab and by no means is the only opinion, I quote though what I know only and practice. This is by no means a small subject, so as the brother before I to must add Allahu wa Alam.