Since you are not marketing a product but your services to build a product they need you have to put yourself in their shoes. They may want an internal program to help with their business, an e-commerce application, a contact database...
You have to think of the common business usage, and then think about that person. They may search for terms such as:
- web designer
- web developer
- internet programmer
- database developer
- contact database developer
- application programmer
They might even be a little technical and heard of a few technologies before so they may search on:
- PHP programmer
- Microsoft programmer
- Access developer
Even though they may call what you do by the wrong name, these are the 'simple' ideas in their head. So you will have to make your marketing website with some of this type of language. Then after that show examples of programs you have created to reinforce that you can do what they are looking for. This would be showing a variety of things you can do to cover multiple situations.
I've tracked leads back from our website for people who searched for 'web designer' but needed a complex internal database. What they wanted it web based so 'web designer' is what they thought of... but I don't think you could Photoshop/HTML an online database ;)
You should always market yourself as a business. For complex applications it is comforting to buyers to know that there might be more than just a single person responsible for the success of their project. Plus down the road if you build up something that can be sold it's easier to sell "ACME Corporation" then it is to sell "AngusWebDevelopment.com"