Physical dimensions, file size and formats.
The most common sizes for banner ads are 468 x 60 px and half of that (234 x 60 px). Other popular sizes are 460 x 55 px, 392 x 72 px, 125 x 125 px, 120 x 90 px, 120 x 60 px, 88 x 31 px, 120 x 240 px.
Publishers prefer small file sizes and often put a maximum file size limit of 10-15K on banner ads. That doesn't seem like a lot but this new Bios font banner made with Adobe ImageReady is only 4.1K!
GIF is the most common format, static or animated, and will cause the least problems. Try to avoid high color depth images with subtle gradations.
Flash banners can work well allowing subtle gradations and animation but not all publishers permit them. Flash renders small type very poorly - consider using pixel fonts to keep small text sharp. This animated banner ad for Nano Sans is a mere 5.8K.
JPEGs are good for full-tone photos and illustrations but are unlikely to display type well at any size. Try splitting such a banner into two (or more) files - a JPEG for the photo and a GIF for the type - but check that the publisher can accommodate such an arrangement.