Technically speaking, visualization is a subset of imagination. Psychic visualization is about giving imagination (which can be purposeless or hazy at times) a more focused, more structured form.
The term 'imagination' is generally used in a broad context; daydreaming, fantasizing or the habit of avoiding reality can give this term a negative connotation. By using the term 'visualization' we specify that we are actively using our imaginations for a specific purpose.
In this context, 'visual' is not only related to the physical sense of sight, but also to psychic sight which can include all other physical senses and beyond.
In its most essential form, psychic visualization is used to bridge the present with the past and future. Reality is a flowing thing and by psychic visualization we determine which way it flows.
For practice, just start with imagination. As you get better, give it more detail and control it with more influence. Even with only a little practice you avoid getting lost in life and you will get from where you are to where you want to be - because you can visualize the road.