There are three models of the ipod touch 1. fourth genreation 2. four s. genereation 3. fifth generation They all take pictures, but the quality of the pictures improve as you increasese in the models. the fourth s. and the fifth both have siri, if im correct. The fifth genreation is a lot lighter and has six rows of apps instead of five. If your thinking about getting one, i'd go for the five. It's really worth it.
There are numerous differences between the iPod touch models. For instance, the fifth generation has six rows of apps, while earlier models have five. Some models have cameras, but others don't. Check out all their differences on support.apple.com/kb/HT1353.