if you search my posts for ipad app mentions, you'll probably turn up a few threads 
essentially, an ipad is a touchscreen computer, yes. except you cannot use any Flash based programmes on the ipad (desn't suport flash).
they are fantastic, and have helped dd1 a lot.
the tracing apps have really helped her come on with her writing - a lot of early years approaches use tracing motions to help young children with the shapes of letters, and this is no different.
dd1 has apps for letters, phonics, maths, games, reading (storybook style, as opposed to letters/sounds). she leafs through holiday snaps (and I use them to customise into storyboards for her to read too), watches films - honestly the possibilities are endless.
if an ipad is not possible due to finances, an ipod touch might work - same functionality, smaller screen, which might be ok depending on fine motor control. dd1 started out with mine, progressed to an ipad, and now uses the ipod on eg days out where I don't want to lug her ipod around. dd2 (only 4) only really gets to use the ipod (as the ipad was fundraised for by dd1, and is hers from school)