“Online” in Business means that a visitor of a website is able to use the functions that are offered through the same website. Examples are:
A contact form or the subscription to an e-mail newsletter. A visitor selects and provides the required input and the system will send the information to what is called a back-office. Often this is the end of the online interaction.
A real-time web environment is said to be transactional. Every action through the website is managed as a transaction in which there are always two sides; the user or visitor enters data and the system responds with immediate feedback.
Web-based financial trading systems is often real-time but not completely. A visitor selects order data in the order screen and this order is routed to the exchange. The system behind the order-entry will immediately verify the financial means that justify the order and after these checks, the order will be forwarded to the exchange.
Yet, still today in 2020 there are still banks that provide portfolio information with the closing quotes from the previous day. That is an online information and not real-time accessed from the stock exchange or another information provider.
The main difference between online and real-time is the interference of a person behind the web application to check for a certain situation. In a real-time environment, there is no human interference.
Another difference is the required level of investments to cater to a real-time application. Online functions are relatively less expensive but at the same time block large amounts of traffic too.
Take for example the functionality behind comments. In an online environment, human interaction is required to validate whether there is no Spam or other misuses of the function. To solve this check-in a real-time environment you need software that checks for Spam and other junk entries. That requires an investment, but at the same time will make your website more scalable for large amounts of visitors who leave comments.
There is a situation where there is (nearly) no difference between real-time and online. This is with chat functionality when you are to address your question about a product or service to a service agent who is currently ‘online’. Nearly, because it might take some time before the agent will respond to your question. Again depending on the amount of traffic and the capacity (availability) of the agents.