This is the original Cessna Citation series, having straight wings and cruciform tail plane. The horizontal stabilizer has a V shape, with a noticable dihedral. The cabin windows have a characteristic Citation shape: a vertical ellipse, with the top and bottom cut off. Also note the relatively large cockpit windows.
The Citation Mustang looks like a small CitationJet, but there are more differences: the Mustang has ventral fins, wide, oval cabin windows and different cockpit side windows, like that of the Premier I.
Take the fuselage and the wings of the Citation I, put a T-tail at the back and different engines, and voilà, basically you have a CitationJet, the first and smallest member of the CitationJet/Citation CJ series. Other members of the Citation CJ family mainly have a longer fuselage, but also other changes compared to the original CitationJet.
The Citation CJ4 is the only Citation CJ family member with swept wings, albeit only very slightly. All others have straight wings. Additionally, the cockpit side windows of the CJ4 are higher.
The Citation Excel series is essentially a crossbreed of the Model 750 Citation X and the Model 560 Citation Encore. The former supplied the fuselage, which was shortened. The wings and tail are of the Citation Encore.
The Citation III was the first Citation with swept wings and a T-tail. It can be distinguished from similar bizjets by its cockpit windows, squared off oval cabin windows, large thrust reverser fairings and bullet fairing on top of the vertical stabiliser.
The Citation Sovereign could be regarded as a stretched Citation Excel. However, it is a lot bigger, the wings have a slight seeep angle and the cruciform tail looks a like that of a Lockheed Jetstar.
The Citation Latitude is a new design, although some elements in the external appearance look familiar. It looks most like a shortened Citation Sovereign, with bent-up wingtips and the delta fins of the Citation Excel. There are only four cockpit windows, so not looking like the six originally developed for the Citation III.
The Citation Longitude has the four cockpit windows of the Citation Latitude. The engine nacelles have no external fairings for the thrust reversers. The T-tail has a dorsal fin with a small air intake at the front. Typical are strakes under the horizontal stabilisers. The wings have curved winglets.
Being at one time the fastest civil aircraft on earth its sleek lines are recognisable. It looks like an enlarged Citation III with huge engines, long, slender wings, a large wing-body fairing and a large bullet fairing on top of the vertical tail, pointing backwards.