You wander around finding clues/pieces of evidence and questioning/interrogating people (suspects, witnesses, etc).
This is the fourth CSI game in the franchise but the first to use a more 3D way of looking at things (still not as 3D as many games but you can pan and zoom most locations - the first three games you saw things as 3D but had no capability to move at all, apart from focussing on hot-spots).
I enjoy this kinda stuff (hence owning all four games).
You actually don't need to find *every* piece of evidence to complete a level, merely the key ones. Hence, I have finished this level but just want to "complete" it before going to the next one (otherwise I would have to redo it from scratch).
Likewise, the people who make the walkthroughs are just experienced gamers and may not find every tiny clue...
Hope this helps,