Guild made the T-100s first in the late 50s, and they then introduced the Starfire line in the early 60s. The SF Is, IIs, and IIIs were built from the same hollow thin-line superstructure of the T-100. T-100s had laminate maple back, sides, and tops. When the Starfires were introduced, I think the idea was that they would have fancier appointments and hardware. Some of the first Starfires were also of laminate maple, but they then became more commonly made of mahogany throughout the 60s. Also, there is the difference in binding as parker points out. T-100s were discontinued in the early 70s. I think the Starfires continued to be manufactured to the mid-70s but were reissued when Fender acquired Guild.
As described by Walter above, T-100s came equipped with one or two single coil PUs, ranging from Franz, Mickey Mouse, various Dearmonds, and other less common types. Early Starfires had Dearmond Dynasonics (and a few other single coil types) and eventually became consistently Guild mini-humbuckers.
There are lots of variations with hardware in the early periods, but the colors were more consistently from a specific palette of finishes, most notably the classic cherry red. We've also seen sunburst, green, black, amber, and natural mahogany too, if I recall.
The Starfire IV, V, and VI are thin-line semi-hollow bodies with humbuckers, which soon followed the I, II, and III.
That's just a quick summary. As is the case with Guild, there are lots of one-offs, customs, and exceptions to these descriptions. There are changing variations in appointments and body materials throughout the history of this line from different factories, and with changing ownership. The best way to get a comprehensive rundown is by grabbing a copy of Han's book, The Guild Guitar Book.