If it's a council parking ticket:
First - it gets doubled if you don't pay it quickly.
Second - you get a threatening letter telling you to pay it for a while.
Third - you get a court summons.
Four - the court sends bailiffs to your house to make you pay the fine, plus the costs you've incurred by not paying.
That is, unless you have a valid reason for not paying. Like, the ticket was unfair. It's actually incredibly easy to get away with parking offences e.g. if the yellow lines are broken at all, if the parking restriction sign is defaced in any way.
If it's a "private parking ticket" given to you by a private company, then you don't have to pay at all. They have no powers to fine you. Any civil claims they want to make have to be done through the courts. This includes: if you have been clamped, and there was no reasonable and obvious warning of clamping (including if the sign was unreadable i.e. too far away...so easily debateable), and instances where they have paid the DVLA £2.50 for your details after noting your number plate, and then sent you a bill in the post.