There seems to be lots of confusion regarding electronic knitting machines, so I figured an article explaining what it's all about would be a good idea!
First and foremost, an electronic machine does NOT do the knitting for you. You will still have to move the carriage side to side manually. If you want to automate that process, you need a drive or motor. They are like hen's teeth to get hold of, and they mount above the machine on a big frame, and you hook the carriage handle up to them. I've only ever seen the Silver one for sale in person once, and it was £1000. Brother had something similar but they're only rarely available second hand. The Passap E6000 can have a drive fitted which extends the machine by a good 600mm on the right hand side, and the back lock plugs into it. I'm thinking of the Electra 4600 here, there are other earlier models I'm not familiar with. Again, it was sold seperately, and is very hard to get on its own.
An electronic machine is identical in every way to the punchcard machine except that it can knit pattern repeats other than just the usual 12st, 24st, or other mechanical variant (depends on your machine). So if you wanted to knit a stitch pattern that was a 5 sts wide repeat, you could do it easily electronically - it doesn't divide easily into 24. If you want to knit a design that is 200 stitches wide, you could do it as long as you have that many needles (and that much patience - I'd be using Designaknit here). Obviously, the next question is how to program this in.
Brother (aka Knitking / Compuknit in the US)
Some Brother electronics came with built-in mylar sheet readers. This is a plastic sheet, usually 60 sts wide, that you colour in using a special black ink pen. The machine itself would read this sheet in and memorise it to its memory or RAM. You could put more than one design on the sheet if you wanted to. You could also split larger designs into columns and rows, and program it in a particular order. As long as the machine has enough RAM, you can program pretty large patterns. Early machines only had these readers, and they could be quite temperamental. The mylar sheet reader read black or white squares.
Any Brother electronic with i in the model name, has a special port which can be used to connect to both the Brother PPD (a programming device) and the modern Designaknit (DAK) software via a special cable. The KH950 and 965 came in both variants, and an upgrade kit could be bought to convert them to the i (replacing the whole electronics part and adding that extra port).
The Brother KH950i socket
A PPD
PPD onscreen display
Floppy disk drive
Some Brother electronics came with cartridge ports. These are along the lines of the old console game systems - you could use the PPD to program the cartridge which has some memory in it, and then plug the cartridge into the machine. The PPD itself is basically a very early computer, with cursor keys and black and white buttons. It connected to a TV via the analogue aerial port, so you could create patterns on screen and save them to the cartridge - there are even rudimentary copy and paste functions, though it's very tricky without a mouse. There was also an additional disk drive (FB100) which used a very early 3.5" floppy disk format. I think it may have been possible to connect the FB100 directly to the knitting machine if it was an "i" model, otherwise you had to load the floppy disk designs onto the cartridge via the PPD. You could read designs TO and FROM the cartridge using a command line style screen, and you could also transfer patterns TO and FROM the knitting machine memory itself.
The KH965 is also on-the-fly programmable, ie black and white buttons and cursor keys.
All of the later Brother machines had built-in libraries of stitch patterns, and they are programmed as if on a sheet of graph paper, exactly how they appear in the Stitchworld books. This means you can use part of a pattern, or even some of the pattern and the white space around it, to generate new patterns. These machines also had pattern variation buttons, allowing you to mirror, change the height and/or width, or invert the colours, and it's also possible to split a 2 colour pattern for double-bed jacquard.
Silver (aka Knitmaster, Studio, Empisal, Singer depending on locale).
Silver machines were a lot more modular. They never came with built-in stitch libraries. Early ones had pattern variation buttons, eg reverse direction, double-width/height, mirror etc, that would work with mylar sheets that are very similar to the Brother ones except they are printed in red, not blue (and the start line is slightly different). The later ones (eg SK840) are completely "brainless" - the only electrical part about them is a cable and its socket on the carriage. This means that they need other additional devices to program them, otherwise any patterning is manual.
The EC1 looks like an old dot matrix printer, except it reads 60st wide mylar sheets marked in pencil (this device reads shinyness). You ran it, powered, alongside your knitting machine. It has the variation buttons as above, and it's basically an electronic version of a punchcard, reading line by line as you knit. This could be further extended with the addition of a PE1 (Pattern Extender). This took a special memory card, which you could read and write to, and has a 5 line LCD display that meant you could double-check any pattern read into it from a mylar sheet. The PE1 is no use on its own, it only worked when plugged into the EC1. A later development was the short-lived PC10, which combined the EC1 and PE1 in one box - it also contained all the original Silver mylar sheets programmed in. However, a major downside was that there was no way to connect it to anything modern data-wise (eg USB or DAK) - the only media it took was an old compact flash card format. Needless to say, it flopped!
EC1
PE1
PC10
You can opt to send patterns only with a Silverknit box - this uses a small piece of software to transmit stitch patterns ONLY directly from your computer. You can also purchase a Silverlink cable - this is the cables to talk to DAK, plus a silver box which is the memory needed to store the information. This is why the Silverlink cable is more expensive than the cables for the other machines.
I'm not going to cover Superba, Toyota or Passap/Pfaff here because I'm not familiar with them. The electronic machines can all be used with DAK as long as the correct cable is purchased; most punchcard machines can also work with DAK but only for garment shaping, the patterning remains mechanical. However, if you don't want to take the DAK route there are other options such as the AYAB board, arduino, Img2track and other "hacks".
This is a brief explanation of various options, the dotted line is because the PE1 is optional. Cables don't neccessarily download AND upload, it depends on the model.
enthralled
exhausted 
lethargic 
