What is the order in which a report gets processed

Compilation: This is the first step, as and when report is developed and you try to run it, it gets compiled. Compilation simply tell you whether there is any issue related to code/expression written at report layer. It saves a staging compiled component at report server.

Processing: This is real task where your data set executed against the data sources it has created and generate the data.

Rendering: Render means to convert the data into report specific layout. It simply showcases your complete report based on browser you wanted to view it in.