A script is the necessary software which is required by a (PTC) program in order to run. In this case ads, referrals (bots), balance, instant payments (if supported) etc will be controlled by the functions a script has.

Many programmmers created various PTC Scripts. These scripts are something like a PC game: Release, software is "on stores", legal sales (10%) & and illegal distribution, piracy (90%). Some programmers are thinking how to scam and that's why placed hidden backdoors to these scripts. Using these hidden backdoors, they could easily gain access to the admin panel and empty AP/PP account. Distrubutors of illegal copies can place of course extra backdoors.
Most PTC sites are not safe and against autoclicking attempts: Autoclicking software is updated often, in contrast with the majority of PTC sites, in which script is not updated because owners don't care/don't have money/don't know. All PTC scripts are not advertiser-friendly. There are tricks, which can help a member to avoid viewing an ad.

Most famous scripts (2010)
Buxhost: A big LOL is enough. With $25/month kids (aka "trusted" admins) can start their own PTC in 5 mins.

Gen2/Gen3/Gen4/Gen5/.../Gen[i] by ptcpay: Gen[i] means that you have to expect hundreds of new scams which will attempt to steal your money - buxhost will contribute a lot of course. From now on - and officially - Gen series will offer a great bot script for better scamming experience. Just in case you have doubts about bots...

* It is a bit outdated but until now nothing have changed, only names. Scamming philosophy remains the same.