Web site development
Web development is the part of the web site build process where all the intricate calculations and decision making is coded into the site. In essence, web development is the same as the network of cogs, pendulums, weights, and mechanisms in a grandfather clock - without them, the clock would look very pretty, but it would not serve any purpose but to stand there. If you solely want people to look at your web site without any form of interaction, then you probably won't need to get web development - web design would suffice. If however you want the user to be able to interact with the web site (lets say to complete an enquiry form, survey, or sign up to a newsletter), then you will need an element of web development as part of the build process.
Don't worry about details - you won't need to tell us that you need web development; we'll be able to figure this out if you request a quote and tell us what you want your web site to be able to do. And if we're not clear on what you want, we'll ask you!