The RX-78 Gundam is the first, and most recognizable, Gundam of the franchise. RX-78 premiered along with the 1979 "Mobile Suit Gundam" anime series. Many of the RX-78's features became iconic to the Gundam franchise, with all subsequent Gundams resembling the original in some form or fashion.
The "Alex" is an upgraded variant of the original which debuted in the 1989 "Mobile Suit Gundam 0080: War in the Pocket" OVA. "Alex" served as a bridge between the original Gundam prototype line and the newer RX-178 series of Gundams.
The Mark-II Gundams are a series of three prototype Gundams first seen in the "Mobile Suit Zeta Gundam" anime series. While outwardly superior to other, more common, mobile suits the Mark-IIs have inherently weak defensive capabilities, which becomes a liability as time progresses.
The Mark-IIs receive an upgrade known as the G-Defenser, which improves upon all aspects of the unit, especially defense. With the G-Defenser installed, the Mark-IIs are able to remain competitive with the enemy's advances in mobile suit technology.
"Mobile Suit Zeta Gundam" protagonist Kamille Bidan puts down the keys to his Mark-II mid-season and becomes the pilot of the Z Gundam. One of the most notable differences between the Z Gundam and previous prototypes is its ability to transform between its standard, battle-ready, form and a more aerodynamic mode used for long-distance flight.
The Double Z Gundam is an upgraded variant on the original Z Gundam prototype. Like the Z Gundam, the Double Z also has a transformation ability known as G-Fortress Mode. G-Fortress Mode increases the mobile suit's offensive and defensive capabilities at the expense of speed and maneuverability. The Double Z appears as a mid-season upgrade during the "Mobile Suit ZZ Gundam" anime series.