This is a common question we get with our clients.
Let's first start with the definition from Wikipedia.
In production and development, open source as a development model promotes a) universal access via free license to a product's design or blueprint, and b) universal redistribution of that design or blueprint, including subsequent improvements to it by anyone.[1][2]Researchers view open source as a specific case of the greater pattern of Open Collaboration, "any system of innovation or production that relies on goal-oriented yet loosely coordinated participants, who interact to create a product (or service) of economic value, which they make available to contributors and non-contributors alike".[3] Before the phrase open source became widely adopted, developers and producers used a variety of other terms. Open source gained hold with the rise of the Internet, and the attendant need for massive retooling of the computing source code.[4][page needed] Opening the source code enabled a self-enhancing diversity of production models, communication paths, and interactive communities. The open-source software movement arose to clarify the environment that the new copyright, licensing, domain, and consumer issues created.[citation needed]
http://en.wikipedia.org/wiki/Open_source
What this means typically for our clients is that the software is FREE! Yes, the software is free, but there are sacrifices you make for that access.
Their is no true accountable entity to call upon if there are issues with your open source software. This means that if your blog just won't post any longer, you cannot go to the vendor to immediately fix without cost. In a more realistic scenario, you would have to find someone in the community or an experienced vendor familiar with the platform that you would pay to help you.
Also, open source is typically more frequently a target for malcontents or hobbyist hackers that like to aim their sights on platforms they are intimately familiar with. The open source nature allows folks to get detailed insight into all of its beauties but also its faults.