3G and 4G are for cellular data. i.e: data that goes to your mobile phone mast. The "G" is for "generation" hence 4G is a lot faster than 3G, although both are much faster than the proceeding technologies.
WiFi is the same wireless technology a laptop may use in your home or office environment.
3G and 4G are typically vastly more expensive in terms of cost per data amount than wifi which is usually essentially "free" (it counts towards your normal internet usage allowance, if you aren't on an unlimited plan).
Most phones can be setup to switch to wifi when available, saving you racking up a huge bill for 3G data charges.
When a mobile phone tarrif plan reffers to "data" they mean the $$$ 3G / 4G kind. Whereas when your phone is using wifi, think of it like any other computer on your home wifi network.