Making sure the appearance and experience of your website is the same across all devices. The mantra is "Mobile First". Design for the mobile user first and add adjustments for the desktop user.
Websites or mobile apps that must interact with backend processes to provide your customer with the services they need over the web. This is the next step beyond web development. The focus here is user interaction.
Often businessess need to communicate with third party services or even provide access tby third parties to their own data. The way apps communicate these services over the web is through "APIs".