This is a prison sentence, but where the judge ‘suspends’ it for a period of time (not more than 2 years) on condition that you comply with certain conditions. There are 13 conditions that can be imposed, but the most common by far are unpaid work (what used to be called Community Service) of up to 300 hours of work of benefit for the community, or a curfew of between 2 and 12 hours for a period of time of up to 6 months.
Only sentences of between 14 days and 2 years can be suspended. This was amended by the Legal Aid, Sentencing and Punishment of Offenders Act 2012, prior to which only sentences of 14 days to 12 months could be suspended. This change took effect on 3 December 2012.
If the offence for which someone is sentenced happened before 4th April 2005, then the judge can pass a suspended sentence, but only if the case is ‘exceptional’. If s/he does, no conditions can be attached to it.
Provided that you stay out of trouble and comply with the conditions, then the sentence is not activated. If either of those things happen however, then you can be taken back to court and ‘breached’. At that hearing, if the Court determines that you are in breach, you can be resentenced and that can include a prison sentence of any length up to the period that was previously suspended.