The Palm Treo is a very popular line of smartphone running the Palm OS.
Treo 90 First Treo did not contain a phone.
Treo 600 All modern Treos follow a very similar form factor to this Treo. The plastic flip piece was removed.
Treo 650 Main features added were bluetooth and non-volatile memory so you wouldn't lost all your data when the battery was depleted. The camera was improved.
Treo 700p (CDMA Only) Main features added was EVDO making the internet much snappier and
Treo 680 700p for gsm, but no high speed internet only EDGE and deletes antenna.
Treo 700w(x) (CDMA only) First Windows Mobile Treo with specs similar to Treo 700p and released shortly after it. Released first on Verizon and X model released later on Sprint with memory upgrade.
Treo 755p (CDMA Only) Very similar to 700p except deletes antenna and uses smaller memory card slot. Software was updated a little bit.
Treo 750 (GSM Only) Windows Mobile Treo for GSM without antenna, but with UMTS and HSDPA.
Palm Centro No Treo branding, but similar to 755p/700p only smaller.
Source(s): Palm Treo