Ok.....here goes
You have two options with an iPad to connect to the Internet/get emails etc. all have wifi so if you have wifi at home or are in a cafe with wifi (starbucks etc) then you can surf the web etc.
However, if you want to have a connection whilst you are out and about, in the same way a mobile has a connection, then you need the 3G version. Simply, you sign up with a contract with a mobile provider and they give you a special iPad SIM card to put into the iPad. For example, I use mine with a 3 Mobile SIM card which gives me 10GB of data (more than enough unless you plan to use it to watch TV when you are not connected to a wifi point. With 3G you can also use it when you are travelling overseas though the data costs are very expensive (I ask 3 Mobile to cap my bill at £40 a month so if I do use it overseas I won't see a huge bill).
So, in summary, a iPad 3G is the same as a iPhone but without the phone part
The only other difference between your links is the amount of storage. One is 16GB and one is 32GB. This will be down to what you plan to use it for. For example, I have the 16GB 3G version and I have 3000 songs, about 50 Apps and 5 movies on it. If you plan on using it mainly for movies then you may be better with a 32GB one though you will pay more than the version in your link if you also want it to have 3G
So, the question for you:
1. Do you want to use it whilst out and about (rather than just at home or in a place with free wifi). If the answer is Yes and you are also happy to pay £10-£15 a month for thee privilege then you want the one with 3G
2. Do you plan on storing lots of movies and thousands of songs on it - if Yes then you need to go for he 32GB
Hope this helps
Steve