It's basically a price or product comparison website with a bunch of merchants displaying their products & offers. That's where the name Comparison Shopping Services comes from.
Examples of CSS websites are:
pricerunner.com
shoppingbymoose.com

Multiple merchants send their products & offers through a product feed to the CSS website.
The product feed contains information about the products such as title, price, description, url, images etc.
It's also possible for the CSS website to scrape product information from ecommerce websites without a product feed.
The product feed is usually in an xml or csv format. But it's also pretty common with content API.

All products & offers are usually displayed on a listing page with filter and sorting options.
It gives the visitor the availability to find products from multiple merchants and also compare similar products & pricing.

Usually it also has a search bar to easily find matching products.

When you click on a product from the product listing page you can either be linked directly to the product on a merchant's ecommerce website or you can land on a CSS product page.

Not all CSS websites has product pages, but it they do it will most likely look something like the image below.
The product is displayed with different merchants selling the same product.
In order to purchase the product you will be linked to the merchant's own ecommerce website.
Meaning even with product pages a CSS website is not an ecommerce with option to purchase directly on the website.