Google / other search engine
Google is often the place that people start searching for products they want to purchase online. With the advent of Product Listing Ads e-commerce has become very important for Google, online retailers as well as consumers. Google relies on ecommerce for much of its Pay Per Click Advertising revenue, retailers for much of their web traffic and customers for the best place to find great prices and selection.

side note: more and more people are now starting their searches on amazon. This has a potential of really hurting both google and smaller retailers.

Amazon
Many people are now moving away from Google as the starting point for their online purchases. Amazon can be the perfect place to start your search. It has good prices, selection familiar interface, your credit card and shipping information, product recommendations and more. Even if the price is not the best for some products it is just easier to shop on amazon and not worry about starting a new account at a different retailer just to save a dollar or two.

Social Media
People are both discovering and now purchasing in social media. These can be a perfect platform for some types of products. Sharing products on Facebook, Instagram or Pinterest is popular right now and will only get more important for online shopping as they continue to integrate the ability to purchase the item right into the platform. Social media platforms can be perfect for finding trends or products from people you trust such as friends, celebrities or experts. That said, people are not going to social media to shop and may not be in the mindset for buying the way they are when search google or visiting Amazon.

TV / Real-World
The ability to buy a product online when you see or discover it in the real world or on a video is a changing the way we discover products and how we shop. Amazon lets you scan an actual product and if they can recognize it you can purchase it right then and their. This can be done by using your phone to scan the barcode, the box or sometimes even the product itself. This is about discovery and convenience and could be a great new way to buy the products you see in everyday life with less effort. Some stores have been experimenting with letting people purchase products that are featured in a video, TV show or movie. This is relatively new and will continue to change as the technology, customer expectations and infrastructure evolves.

Side note –  i recently scanned a bottle of organic protein powder and within 3 minutes of scanning it i was able to order it on amazon. A search for organic protein powder would have brought up a lot of other results but a scan of the bottle easily found the product and brought me right to the Amazon product detail page.

Side note – Google recently added a barcode scanner to chrome that takes you to the search results and shopping results for that product.