What is the difference between Wi-Fi and mobile Internet (2G, 3G, 3G+ or 4G)? Surfing via Wi-Fi is cheaper than surfing via mobile Internet, but the Wi-Fi signal is limited to a small area, such as a cafe, airport, etc. To see the differences, you can compare the Wi-Fi Hotspots map and the network coverage map.
Wi-Fi uses a local wireless network to transport data. You can surf the net if you are close to a modem or hotspot.
Mobile Internet uses a mobile telephone network (and therefore your SIM card) to transport data. You can surf wherever you can call with your mobile phone.
With your SIM card, you can use the following networks: 2G (GPRS or EDGE), 3G (UMTS), 3G+ (HSDPA or HSPA+) and 4G (LTE). You may be switched from one network to another (e.g. from 3G to 4G) because your device always chooses the best available network.