Hydra 2 is a graphics editing tool that allows you to create HDR images. HDR stands for High Dynamic Range. Hydra 2 uses a technique by which several images are mixed into a single image. The resulting image looks more real than the previous ones because it looks like what your eyes really see. With Hydra you can use up to ten images of varying exposures (darker and lighter) to achieve this result.
There are two versions of Hydra: Express and Pro. The Express version limits the number of input images that you can use to three. It also doesn't have a trial version and no advanced options or filters. The Pro version, on the other hand, comes with Lightroom filters and it gives you access to both input and output filters.
All the images that you add to Hydra have to have the same size, or they will be skipped. To obtain the best possible results, you would have to include pictures with different exposure times. Since cameras have physical limitations on how much light they can capture, by combining different pictures you can create HDR images.
There are quite a few examples of HDR images on the developer's website, and there is even a screencast that shows you how to take the pictures to achieve the best results.
Hydra is relatively easy to use if you have any experience with graphics editing software.