Shower gel is generally a thick, semi-opaque gelatinous substance. Despite its name, it does not have to be used only in the shower.
To some people, it's for cleaning off dirt. To others, who believe advertising, it's vitamin E-enriched, cleansing, toning, moisturising, soap free, 100% pure, foam bath with added aloe vera and jojoba root... and it will cleanse, improve our smell, take ten years off our lives, improve our jobs, career prospects, and social lives and will add shine to our hair.
Shower gel comes in plastic or glass containers, depending on how expensive it is, and smells of clean. It comes in a selection of colours, depending on its aroma, and the mood which it is supposed to enhance, such as 'invigorating' which is usually blue or 'relaxing' - typically green. You can also get shower gel with bits in it; these bits have no real reason for existing but can add an awful amount to the value of the product.
Shower gel is often used in preference to soap as it doesn't dry your skin. Some people don't use either, they should be avoided.