FTP is a way to get files from your computer to your website. On the iPhone that generally means transferring photos, but at least in the case of one app, you can FTP any file you can load onto your phone, photos are only part of it. These are some of the iPhone apps for FTP that I found in order of price.
PhotoUpLink is a simple photo sharing tool that includes FTP as well as Wi-Fi connectivity to share your photos. It is supported by an unobtrusive ad across the top of the screen. It's easy to use and will connect to most FTP servers, but it doesn't use secure FTP.
This app is free.
The nice thing about this photo uploader is that it will do things like create thumbnails and resize your photos for you.
This app costs $1.99.
FTP on the Go is a great little iPhone app perfect for anyone who woks with files on a server or needs to transfer files between two computers. It acts as both a file storage device, an FTP client, and a text editor. It's so handy that I've written several reviews right in the app itself.
This app costs $9.99.