We are stating the obvious, we know, but since we are explaining what we offer, we might as well.
E-commerce sites are the ones where you purchase products online. These products could be of various nature: services (cleaning, etc), actual goods (books and shoes, for example) or electronic goods (programs, videos, web hosting, songs..).
These sites have usually a different look from your "normal" webspace, as they are designed to ATTRACT potential customers and appeal to them with awesome products and prices. All transactions occur online, using safe gateways and payment methods (PayPal, ApplePay, etc).