Basically, tableless design is where the HTML/CSS programmer, creates a template from a graphic file (like a PSD file), using entirely CSS to layout the “columns” and “rows”. So a two column layout is actually all CSS using DIVs. There are numerous sites that will explain the process and DIVS offer many benefits over tables..
Its also a search engines best friend :) I love CSS because all the design aspects are in a seperate file so your not constantly replication redundant information such as widths and heights, and you can make global changes to your website’s design easily.