The Agility is the budget line, and the Vertex is the performance line.
With the latest generation, they've also introduced the Solid line, which fits under the Agility line, so it's super-budget. Additionally, they have a higher version of the Vertex called the Max IOPS version, which is the fastest of them all. They might also call this the Pro model, not sure.
So you have (from slowest to fastest):
Solid
Agility
Vertex
Vertex Max IOPS