List of fields and explanations?
One way is to look in the MySQL database directly, if you have access to it. Each table is defined with comments for most of the columns, and I have found them to be quite useful. 

A great way to inspect the databse directly is to use MySQL Workbench. You can right click on the table and choose Table inspector. Here's a screenshot of it which demonstrates its ability to display field comments...



Other database inspection tools like phpMyAdmin should have similar functionality.