Locked / Unlocked
A 'locked' mobile phone is one that has been restricted by software so that it can only be used with a single network or carrier.
Locked mobile phones are usually GSM phones that use a SIM card - hence the use of the term 'SIM lock'. The reason for the lock is because mobile phones are usually heavily subsidised and carriers want to discourage people switching networks or shipping phones to other countries.
'Unlocking' a phone means that it can be used with any SIM card/carrier - and in any country. But the unlocked handset may still display the original network's branding and may not support features of the new carrier. Most phones can be unbranded by uploading a different version of the phone's internal software or 'firmware'.
Perhaps the most famous - or infamous - example of a locked mobile phone is the Apple iPhone When it was originally launched it was locked to the AT&T network in the US and O2 in the UK, both of which paid a hefty premium for exclusive rights to sell the phone.