Studio: historically, no binding, lesser-quality top wood, maybe multi-piece bodies, "entry-level" Gibson pickups (496/500s and such). Recently, the "entryu-level" Studios have had a satin finish and have been a bit less expensive (current ones go for $799 new).
Standard: Body and neck binding, better-quality top woods, and generally nicer burst finishes. Pickups are '57 Classics IIRC. Hardshell case included. Now they are starting around $2700.
Custom: Double-bound bodies and rectangular inlays, HSC. Going in the mid-upper $3Ks.
The 2015 line was a total diversion (the zero-fret nuts, wider necks, and G-force tuners), as none of the models had satin finishes, with the Studios going for $1249 with HSC. The Guitar Center/MF "VR" version of the Studios released late in the year came with vintage tuners and the regular-width neck for $999.
Not that I am a huge Gibson fanboy, but there has been a lot of hyperbole over the last few years regarding QC. Sure, Henry is a whack-job, and has made some really boneheaded decisions during his reign of terror, and a few years ago there were some pretty widespread QC issues, but Gibson still puts out some great quality guitars, and the QC has been a lot better the last couple of years. Frankly, a LP Studio of fairly recent vintage used can be had for around $600-700, satin or gloss, and if you grab a good one it's a fgreat value. One of the area GCs had a beat-up used 2012 satin Studio for $499, and it played and sounded fantastic. If I had the spare cash I would have taken it home.