To say that the publishing field has changed is sort of like saying we have a few more cars than we did one hundred years ago. The big change: technology.
In years gone by I would write my great American novel and then try to find an agent. Publishers would rarely consider your work if you weren't represented by an agent.
I would do my research and find an agent who seemed to be working in my genre, then send a synopsis, letter, and chapter samples.
And then wait. more here.