What's the difference between a website and a web application you ask?
First, the lines are being blurred more and more as technology evolves, but the best way to think about the difference is to
see the two as on a spectrum (see image, above).
The Web Application
When you think about a web application, think about GMAIL or Facebook. With each application, you have to login so that the application can authenticate who you are. Then, the application delivers customized content to you.
For example, everyone on GMAIL sees a similar menu, but all the emails are your, personal emails, not anyone else's. You'd be quite surprised if you logged in and saw the same emails that your partner saw when s/he logged in, right? In fact, that might be quite confusing or disturbing!
Similarly, with Facebook, you choose who your friends are and then you see content that is customized specifically to you. This is a key difference between a static website and a web application; in a web application, the content is customized for each user. This takes a lot of programming and a lot of very smart (and very expensive) programming talent to code and maintain the code that performs this customization and the logic upon which it works.
The Static Website
When you think about a website, think about your local bakery or yoga studio.
They have a "static" website with a lot of pages that describe their services. You might be able to send them an email through the website, but the website is similar to a printed brochure - you can think of it as simply a printed brochure that is placed into an online format that describes the products and services.
One key point to remember about static websites is that everyone sees the same site. Whether you are Tom, Dick, Susan, or Harry, when you navigate to your local bakery's website, you see the same content as everyone else. Now compare that with GMAIL - when you navigate to your GMAIL account, you'd be pretty suprised to see someone else's emails! GMAIL delivers customized content to each user. That's a key difference. When you are on Facebook, you see YOUR friends and their posts and the content is unique to you based on your list of friends.
Behind the scenes there is a staff of developers writing code, encoding the logic in a language called PHP that encodes how Facebook is showing your different friend's posts. When you build your bakery website, there is no logic necessary, so no backend coding language is needed to customize the content.
Well, what if you have a private membership area with individualized login codes?
Truthfully, when you add the ability to login to a website you are moving slowly along the spectrum of a website moving towards a web application.
Now, some users can login and see content that other users can't. However, it's still not like Facebook or GMAIL where the user sees very very customized content. If you have 100 members who have access to a members-only area of your website to watch your video tutorials, each of those 100 people are still all seeing the same video tutorials. People who can't login can't access that area, but it's basically binary - the content for members and the content for non-members separated by a pay wall (i.e. people have to pay to get on the other side of a wall to see the members-only content).
Now if you have a Wordpress website with LEARNDASH or another excellent Wordpress LMS (Learning Management System) and it is remembering which videos each person watched and marking them as watched and then emailing them a certificate of completion when they finish the course, well you are getting more and more into the realm of a custom web application since you are incorporating a PHP-based plugin that has a lot of logic and is storing information about each user in a database - these are the kinds of things that web application start to do - serve custom content by storing data about each user in a database.
For most of the clients I work with, they want a fairly simple website and they want to scale to create an automated small business that can scale, but scalability is relative and not everyone needs to scale to 1 billion users - some people are happy making high margins and serving thousands of users - there is no need to have ever online business be a VC-funded scalable startup at the scale of Twitter, GMAIL, or Facebook.
Most of my clients don't have the funds to hire a developer at $120,000+ per year to write the custom logic of their website and to keep them on staff to maintain that complex code.
The "brain" of most automated small business owners sites (i.e. the smart, customization) is coming either from a special plugin (e.g. LEARNDASH in the aforementioned LMS example or, more commonly, they are integrating a few other simple applications that can be customized - such as AWEBER which will deliver custom autoresponders at a pre-determined schedule and can be qued up and delivered in a customized way to each user, and a shopping cart that can conduct credit card transactions and send the user a link to a special area of the website or a link to a digital product download. In fact, our courses show you how to build a simple website but to incorporate a few other smart applications like AWEBER for email marketing, WEEBLY to build an e-commerce website with a great integrated shopping cart, and Facebook or Google's Adwords product to drive new customers to your website in an automated fashion.