Monopoly is the one who rules a particular business without any competitors( may be having over 80% of the market share of the business ). For Example before 1984 british gas was the only gas supplier who served the you K..They may charge howmuch ever they want as they don't have any competitors.
Disadvantage? OK, here you go, here is an example...
Microsoft created Windows, they made it so big and so fast that other company's couldn't keep up, so all the little people lost their jobs and had to try to live without using the skills they work so hard to achieve in life. While the employees at Microsoft made tons of money and are still doing great. Example 2...
Remember all the great little stores that we all used to shop at in the early 90's and late 80's? Well, a monopoly made most of them go out of business.
When Walmart opened their doors they put a lot of other people on the street and without a job.
So disadvantages? Yes, the word Monopoly can't be used without first creating jobless areas.
