You've probably seen these used on Amazon or other big e-commerce sites: once you're looking at a product, you can click on a color swatch (or something similar) to view the same product in another of its variations. While it may have a different product number (or SKU), for the customer, it's essentially the same product.
That's the important part to remember: to the customer it's the same thing. If I'm shopping for shoes and I find one that I like that comes in multiple colors, I shouldn't have to click back to the category page and into another product each time I want to consider another color option.
Grouping product variations together also helps you overcome the issues of optimizing for SEO and optimizing for sales. It can never be said too many time: duplicate content is bad. If those shoes I liked have 5 different color variations and each one is its own product, that's potentially 5 pages with duplicate content (because let's be honest, just changing "green" for "blue" isn't enough to fool Google, and we all know you're not going to draft 5 unique product descriptions for the same damn shoe). Selling multiple variants on a single page takes care of this duplicate content because all the products are (or should be, if it's done right) on the same URL.
Additionally, if each shoe was an independent product and by chance one of them happened to rank, or someone found their way directly to one of them from another source without having landed on the category page first, they might never know that you even carried the other 4 colors. If they didn't land on the color they really wanted, you may have just lost a customer. Having the ability to switch between product variations right on a product page instantly tells a potential customer that they have options when it comes to this product, and all it takes is one click to view each one.
Add to Cart
You may be thinking to yourself "well of course a shopping site is going to have an add to cart button... how else are people going to buy things?!" Well hear me out, OK? Obviously the ability to add things to a cart is an essential part of e-commerce, but more important is how it's done.
You're browsing a site that sells DVDs (or Blu-Ray if you're really on your game) and there are a few titles that you know you want to buy. You've seen the movies, so you don't even need to read the synopses or read the reviews... you just want them. Unfortunately, the site that you're on doesn't have an "Add to Cart" option on the product preview (the listing on the category page), meaning that you're going to have to click into the first movie, add it to your cart, go back to the category page, find the next movie, click into it, add it to your...
Starting to see what I'm saying?
We all love to assume that people are going to read every little bit of content on our sites, and we love to map out the paths they're going to take through our pages, but that's almost never the case. People tend to know what they want, and they want it easily; making them bounce back and forth just to buy things from you is annoying and unintuitive. Imagine having to exit and reenter the grocery store aisle every time you wanted to add another product to your cart!
There's another caveat about Add to Cart buttons, too: add the thing to the cart... and let that be it! What's worse then having to click into each and every product I want in order to add them, is when the Add to Cart button not only adds the product but takes you to the cart to make you look at it, too. If you're truly trying to sell products, don't ever assume your customer is done shopping. Let them see what's in their cart via the sidebar, and maybe display a message telling them the product was successfully added, but never push them towards checkout against their will!
The internet is a strange place when you get down to it, and retail is no different. Walking into a department store (do people still shop in department stores?) you can compare products in your hands. You can hold them and feel the quality (or lack thereof) and make decisions based on a number of things. On the internet, you can't do that, and there's always that little thought in the back of your mind that each new vendor or site is secretly peddling knock-off goods and you're going to get ripped off. (or is that just me?)
Customer reviews can be a great tool for businesses who sell quality products that can be found in other places around the web. If you're not selling quality products and you know it... well then these aren't for you. Also, why bother?
Gone are the days when shoppers seek feedback from others on mostly big-ticket items; the internet makes it easy for people to write and find reviews on even the smallest of purchases. And make no mistake, they do: according to eMarketer, consumer reviews are almost 12 times more trusted than product descriptions by manufacturers and sellers. If you don't feature customer reviews and a potential customer leaves your site in search of them, there's a good chance you lost that business.
Now, having the ability for customers to write reviews and actually having customer reviews are two very different things. They don't come instantly, but obviously the latter can't happen without the former, so building the functionality into your site is a necessary first step, whether or not people initially use it.
The List Goes On and On
There are a lot of little things that help make an e-commerce experience better for users. Some major, some minor, but all important. Rather than ramble on about them all, here are some quick points:
Photo Zoom: Even if you're selling something as basic as pens, chances are someone is going to want to zoom in to take a good look at the clip. In a physical store, buying has a lot to do with touching and seeing; your customers can't touch things on the internet, so don't limit their sight too!
Quantity Selectors: Why (besides a sale) would you ever want to limit the number of products someone could buy from you? Sure, have the default be one, but make it visibly clear that they can choose as many they want/need.
Related Products/Also Viewed: Showing customers a product similar to what they're looking at can be a great way to up-sell or introduce them to other products they might not have considered. This works best on bigger sites where a customer can't quickly familiarize themselves with the entire inventory, but anyone could make use of it.
Seamless Checkout: Nowadays most checkout processes are fairly well programmed right out of the box, but it never hurts to check. Go through the checkout yourself every so often to make sure that all the steps are easy and natural; just because a person is in the middle of the checkout process doesn't mean you can't still lose them and their sale.
There are lots of subtleties when it comes to shopping, whether it's online or in the store. There's a person, somewhere, who makes a living setting up mannequins and displays and figuring out what layouts make people buy more polo shirts. It often falls on our shoulders to implement changes like these when it comes to your site, but no one is going to know your store better than you. Don't just administer your site, use it. Does something seem off? Is there a process that could be easier? Your customers will notice these things, so you need to be aware of them too!