A little description on main and side characters.
MAIN
Your main characters are the main focus of the story. They are the ones leading the story and the ones that need the most planning and creating.
They need to be original and different, with personality traits, flaws, likes, and dislikes. The main characters are the most important parts so you need to spend more time on them than your side characters.
SIDE
Side characters add more substance and depth to your stories, creating a world that is livable and interesting.
If you are going to add side characters, which I highly recommend you do as they add more depth and substance to the story and your main character(s), you have to make sure they matter and serve a purpose. If you have a bunch of side characters that are only mentioned once, there isn't a point. They need to have a role and sometimes even their own side plot to make them necessary.
Subplots support your main plot and help it along. I also really like subplots that are used as Easter eggs.
They add layers to your book in ways that the main plot cannot. The only problem: don't use more than three or there will be too much going on and it will overwhelm the reader.