You've set up a configuration for a performance session and started it. While your application is running, dotTrace collects performance data and writes it to *.tmp files located in the user temp directory. After the profiling session completes, gathered information is displayed immediately.
dotTrace Performance provides several views. Each view gives an opportunity to look at data from a different angle. By default, the Overview view is active.
You can work with a snapshot and its data even if it hasn't been saved yet. However, by saving it explicitly you can store not only performance data, but some contextual information as well. For example, you add some details about the snapshot as a description, provide your comments for functions, etc. Next time you open the snapshot, the information and all modifications, if any, will be taken into account and displayed appropriately.
Performance data is stored in a *.dtp file. Contextual information is stored in a file with *.dtp.DotSettings extension. A profiling snapshot is essentially a combination of these files.
The size of a snapshot file is proportional to profiling time and complexity of a profiled application, so it can vary from a few megabytes to several hundreds of gigabytes.