There are many different banner types, so it is important to understand the differences.
Static Banner Ads
This is a banner advert that is static. It's usually in a simple file format, like a .jpg or a .gif (different types of image files), and it has no animation whatsoever.
Pro: Easy to build
Con: Boring, easily missed, limited space to fit information in.
Animated Banner Ads (part 1)
Animated banner ads can often be .gif files. The way these work are that they serve a rotation of images that give the impression of animation.
Pro: Easy to build, common file type.
Con: Not interactive, not aesthetically pleasing, can look amateur.
Animated Banner Ads (part 2)
Other animated banner adverts could be .swf files (flash files). The animation is much smoother and runs more like video. They can also include interactive features, to engage the user.
Pro: Engaging, professional eye catching banners
Con: User needs flash player installed, more difficult to build
If a professional is building your banners for you, then it is good practice for you to have an idea of what it is they're building. This way you have an understanding of what you are potentially paying for.
Banner Size
To start with banners need to be built in certain sizes (width x height), these are measured in pixels and there is an industry standard. Different publishers will show different creative sizes, so most artists will create your banner in each size.
300 x 250 IMU (Medium Rectangle)
180 x 150 IMU (Rectangle)
728 x 90 IMU (Leaderboard)
160 x 600 IMU(Wide Skyscraper)
300 x 600 IMU (Half Page Ad)
120 x 60 IMU (Button 2)
88 x 31 IMU (Micro Bar)
Banner Weight
Most publishers also specify a certain file size, these are measured in bytes. As the web page loads, so does the banner file itself. This means that the heavier the file, the longer it takes to load.
Now there are recommended maximum initial download fileweights of 40KB for all file banner sizes, except the following two:
120 x 60 IMU (Button 2)
88 x 31 IMU (Micro Bar)
Banner Animation
Most banners now include animation, but any strobing colour is often restricted by publishers in recent times. In addition, a lot of publishers request that animations last no longer than 15 seconds, and end on a static image.
Hopefully that's covered the basics of building a banner advert and should help you to understand a little more about what's required.