Mobile broadband is a type of internet used for mobile phones, laptops and tablets such the iPad when there is no WiFi or landline available. It is also known as 3G or 4G internet. 4G provides internet with speeds many times faster than current WiFi internet, however it is not currently widely available in Spain. Still, for most people's needs, 3G offers acceptable speeds.
At the moment there are two main ways that mobile broadband can be provided: through an external USB modem (also known in the UK as a 'dongle') or similarly, through a mobile hotspot device such as the 'Mi Fi'. The second way is or through a data SIM card and used together with your current device.
Dongles can be plugged into the USB socket of your laptop like a USB pen drive and will provide internet service to your PC or Mac. A Mi Fi works similarly but instead of plugging it into your computer, you simply turn on the 'WiFi hotspot' and then log in with whatever WiFi enabled device you have. Both dongles and Mi Fis have SIM cards inside them, the same as what you would find in a mobile phone, and run on a carrier's mobile network. Very often these are provided by the mobile network operators and can provide internet at the same speed as ADSL wherever there is coverage. In Spain, dongles can be purchased on an 18-month contract, usually with unlimited internet, however some operators do offer a 'pay as you go' option which can be slightly more expensive with a lower internet allowance. This is ideal for travelers who are visiting Spain for shorter time frames.
For 3G internet on mobile phones and tablets, such as the iPad, you will need to buy a SIM (or micro SIM) card from a Spanish mobile network provider and then get set up on a data plan. For phones, the SIM can be bought either on an 18-month contract, or on 'pay as you go' with both options usually including a certain number of minutes and texts to use in Spain. The vast majority of travelers prefer pay as you go services with no commitment. You'll need to make sure that your phone is not locked to a network before coming to Spain if you wish to use a Spanish SIM card. SIM cards for tablets are also available on contract or 'pay as you go' from the mobile network providers, however can be a little harder to find and a little more expensive than for phones.
Pay as you go mobile broadband is ideal for people who need access to the Internet while in Spain but don't need it all year round and so don't want to get locked into a contract. It's also perfect for people on the move, or for those who can't get a land line in their house.