There are many different sects within Islam [it is hardly monolithic]. The sect which resembles your term "Islamism" is the Wahhabi movement. It was started by Muhammad ibn Abd al-Wahhab in the smaller city states of Arabia in the 1700s. His extremely traditional group opposed all innovations within Islam, often using violence to enforce its views.
Today Wahabaism is quite strong in Saudi Arabia [where interestingly enough most of the money funding the terrorists is coming from]. It demands punishment for those who enjoy any form of music except the drum and severe punishment up to death for drinking or sexual transgressions. It condemns as unbelievers those who do not pray, a view that never previously existed in mainstream Islam.
Osama Bin Ladin is a member of this sect.
Thank you JMJ. Have you seen JasonC's offerings on another thread? Good explanations to be sure.
Here's a thread where JasonC offers a good deal of information ... for your reading; I claim neither agreement nor disagreement, it's just more grist for the mill.
http://www.freerepublic.com/forum/a3ba2f3210d1e.htm ... the URL