To help answer your questions;
HDS 5 is a sonar/chartplotter combo unit. This is the original HDS unit.
Elite 5 is a sonar/chartplotter combo unit. Fewer features than the HDS unit to hit a less expensive price point.
HDS 5 Gen 2 is the next HDS unit. Same as the first generation but this new unit has a faster processor for quicker page loading and refresh rate. Plus a new feature called Structure Map. This is a sonar feature and only applicable to those who have an LSS-1 or an LSS-2. (LSS = Lowrance Structure Scan)
As mentioned in one of the other posts, Lowrance makes a dedicated Chartplotter without any sonar features. These units will have an “m” after the model number. Example HDS 5m, Elite 5m or HDS 5m Gen 2.