There are three main tablets groups - Android, Apple and Windows.
They all run different software and a program written for one platform cant be run on the other platforms.
If your wife already has a smartphone it makes sense to buy a tablet using the same technology. So if she has an Android phone get her an Android tablet and so on.
Apple are the most expensive, though they are well made and very easy to use. Apple has the most "programs" (apps). Apple makes both the hardware and software so they are well integrated, but you pay for the name.
Android software is provided by Google and is used on dozens of tablets by lots of manufacturers, Samsung, HP, Dell, even Google themselves make tablets.
Android is also easy to sue, and while they don't have as many Apps as Apple they still have thousands and thousands of them.
Android tablets tend to be cheaper than Apple, through the "quality" Android tablets from companies like Samsung can still be expensive.
Tesco Hudl (the latest is the Hudl 2) is only about £129 and has been well reviewed.
Microsoft got late into the tablet market and are playing catch up with their tablets. Personally I would not buy a Windows tablet at the moment, maybe wait until Windows 10 comes out next year which may work better on tablets. There are far fewer Apps for Windows than for Apple and Android.
Probably the most important thing is screen size. I have a Google Nexus 7 with a 7" screen which I find a little small.
The Hudl 2 has an 8.3" screen I think ti is and when I saw one in Tesco the other day it looked a decent size.
You can also get 10" tablets but maybe they are a little big for some people.
If this is her first tablet then I would suggest the Tesco Hudl 2 as it is not that expensive (and can be even less with a Tesco card) and if she loves it then you can get a "better" one in a couple of years.