Can you text people with an iphone if they have an IPOD TOUCH?

Im getting my iphone in a couple days and some of my friends text with their ipods so can i text them on my iphone of they have an ipod touch and also will the texts send if im shopping or something and their at home? (sorry if it doesnt really make sense)

There are texting apps you can use on any phone or table that is connected to WiFi.

In my opinion the best way is using Google Voice.