It is increasingly common for companies to request that applications are filled in and submitted online, which means that you will have to spend a considerable amount of time thinking carefully about what to include. You should consider the application form and your CV as one and the same thing - both are platforms for you to showcase yourself and stand out from the crowd.
Making a good application depends on identifying your key skills and being able to demonstrate concisely how they fit in with the requirements identified in the job description.
You should also think about how this position (and you, if you are to fill it) fits into the company as a whole. What is their mission statement? What are the company's short-term and long-term goals? How can you demonstrate that you can help them get there?
Think about how this position fits into the company as a whole. Remember that you only get one chance at an online application, so it's worth the time it takes to fill it in correctly.
Your local careers service should have examples of these applications, or you can print a few off the employer's website for practise.