Here are the basics on getting some of the most popular of these programs.
Adobe actually supports Linux these days for many, but alas, not all of their programs. While proprietary, the Linux versions of Adobe Flash and Acrobat Reader work fine and are easy to install. Indeed, Reader is as simple to install on Linux as it is on Windows. With the Flash Player, you should read the Flash installation instructions carefully to make sure you do it right.
The one caveat is that Flash Player only runs natively on 32-bit operating systems. If you're running 64-bit Linux, or 64-bit Windows or Mac OS X for that matter, you'll need to jump through the hoops detailed in Adobe's Flash Player on 64-bit operating systems technical note. None of this is hard, but if you go without knowing about these concerns before hand you can end up wasting a lot of time troubleshooting them.
Commercial DVD Discs:
I bet you think you own your DVDs don't you? You don't. Their content is 'protected' by DRM (digital rights management) software that makes it difficult to make copies of it come the day your three-year old feeds Toy Story to the dog or to even play it on a non-approved system, such as your Linux PC.
You see, vanilla Linux doesn't support DVD video playback for two reasons. The first reason is that most commercial DVDs are trapped by the content scrambling system (CSS) encryption DRM. Cracking a video to play can run afoul of the Digital Millennium Copyright Act (DMCA). This has the effect of making it legally questionable to play your DVD on your PC unless it's running approved software. On top of that, the content itself is kept in the MPEG-2 video compression format, which is protected by open-source unfriendly patents.
There are ways around this. First, you can buy a program that will let you legally watch DVDs: Fluendo DVD Player for 19.99 Euros, or about $30. Some vendors, such as Dell, also include Fluendo DVD player functionality in its Dell Ubuntu laptops and netbooks. What most people do though is track down Fluendo's DVD codecs, or other DVD player codecs on third-party software repositories and install this functionality for free.