simple explanation of a crispr screen vs pooled crispr screen?
In the context of pooled CRISPR screen, you first design a library of gRNAs against a certain set of targets (instead of just one target). 

The library can have the size you want, most commonly it is either targeting the kinome, or the whole genome (genome-wide).
In this context, you would add the full library on your cell of interest in which you want to make the perturbations.

Depending on how the screen is set-up you try to select the perturbations which either allow you to pass a selection process (toxicity, fluorescent readout, etc). Usually an elegant way is to have either a label also code barcode which helps you to identify the gRNA which generated the surviving/selected phenotype. With the barcode/gRNA you can track back which gene was likely perturbed in the screening process.

Hope that helps and that answers you question, please don't hesitate to further refine it if this is not clear to you.