There are a number of versions of Dreamweaver available but they all effectively perform the same or similar functions. It is a professional HTML editor for designing, coding and developing websites, web pages and web applications. The application offers 2 design environments in which to work, the code view and the design view. You can also select a 'split' view that lets you see both the code and design views simultaneously.
The design view (visual editing features) allows you to very quickly create web pages without the need to write the associated code that lies behind the design, Dreamweaver takes care of this on your behalf. You can drag in and position the various page elements from an adjoining panel and streamline the development work flow by creating and editing images in third party graphics applications such as the aforementioned 'Photoshop' or Adobe's 'Flash' program.
What you will find in practice is that although you can in theory design a website completely from the design screen that you will in fact combine both functions to produce your designs more effectively i.e. the basic design is drawn together using the visual editing features and then the coding view is used to fine tune your design to complete the final details. From experience, it is likely that as you become more familiar with the program that you will find yourself utilising the coding functionality more and more, particularly for managing the pure code aspects of the design such as CSS functionality (cascading style sheets).
Photoshop also comes in many versions and forms, but again the basic functionality is very similar for the different versions. Photoshop allows you to configure and manage static images such as digital photographs or generated images that you can create within the program such as the example included above for a web hosting graphic, which was created completely using the Photoshop Elements option .
One of the key benefits of Photoshop is that it is relatively easy to use and has been developed with website integration in mind. You can create and save files for the web, reducing resolution for fast loading on the Internet whilst retaining the quality of the image to a level where the differences to the original file are for all intents and purposes indiscernible.
It goes without saying that in order to benefit from any of the programs mentioned, any budding web designer should consider taking a web design course to at least get the basics sorted out. My recommendation would be that any courses that are undertaken would include what I consider to be the two core industry standard software packages mentioned and ideally would also include a basic course on the use of Adobe's 'Flash Program'.
There are also many free courses and lots of information on the Internet for almost every topic and once you are past the basic principles and standard practices stage of your development you will find that these courses plus practical experience will prove invaluable for further development of your design skills.