Microsoft Project is a very popular, industry wide Project Planning tool that comes in 2 basic forms, Standard Edt and Professional Edt. As with most Microsoft products, there are various differences between the 2 versions that can make all the difference when deciding which one to buy for you & your company.
One of the main differences is collaboration. Standard is a stand alone product for Project Managers that do not need to collaborate and work with others on projects; while Project Pro allows people to collaborate across enterprises via Project Server (which is a separate product).
The Microsoft Comparison Chart shows the features of the different versions (including Web Access) side by side to help make your decision a little easier.