480i stands for an interlaced resolution of 704x480, and 480p stands for a progressive scan resolution of 704x480. Interlaced resolutions are made up of 60 partial frames per second, and progressive resolutions are made up of 60 full frames per second.
Each interlaced frame consists of stripes that alternate between the picture and blank. The total of the non-blank stripes is equal to half of a frame. Interlaced resolutions are used to save data bandwidth.
The 480 resolution is considered standard definition, while 720 and 1080 are considered high definition. The progressive scan and interlaced designations also apply to high-definition resolutions.