Angiosperms can be categorized based on the number of their cotyledons, or the first embryonic leaf.
For the history of the two classes:
Dicots (Magnoliopsida) and the Monocots
(Liliopsida) as well as the additional traits and definitions to differentiate the classes.
In a study using chloroplast genomes, Chang et al. 2004 suggests that from bryophytes, plants evolved afterwhich, monocots separated from dicots based on the genes that were present/absent in the genome