A SIM only deal is similar to a standard mobile phone contract, except instead of getting a new handset with your purchase, you simply get the SIM card. You then pair the SIM with your own handset, and you’re ready to go.
By not having the cost of a handset to pay back through your monthly line rental, SIM only deals are generally much cheaper than mobile contracts. Heavy data users can enjoy large monthly allowances at lower monthly costs, alongside shorter contracts.