Here are the primary features/differentiating factors in MacBook Pro across various generations:
Source: MacBook Pro - Wikipedia
A MacBook Pro model (or any Mac, generally speaking) is commonly recognized with an identifier of the form Early/Mid/Late Year based on the time of the year it was introduced. For e.g. Late 2013.
You can use Mactracker, which is an excellent reference app, available for free both iOS and macOS. It lists details for all the MacBook Pro models (along with all other Apple products) ever released. It lists their features, hardware specifications, current production status and supported macOS software versions. Mactracker is available for download via the respective app stores:
2nd generation MacBook Pro models are much more ancient, with the last one released in 2012. Most of them no longer have the ability to run the latest macOS software.
3rd generation MacBook Pro, which were released between 2012 and 2015 feature external design similar to the 2nd generation along with similar design for the keyboard. Prominent feature of this generation include presence of retina display, thinner and lighter form factor, removal of LAN port and Super Drive etc.
4th generation featured major external design overhaul, along with a new keyboard design dubbed the butterfly keyboard. This generation has also seen removal of USB-A ports, card reader and Thunderbolt 2 ports which are all replaced by USB-C ports.
The specifications and features for all the various models of MacBook Pro released are listed thoroughly in the linked Wikipedia article. In conjugation with the Mactracker app, it is the recommended go to place to compare between various generations and models of MacBook Pro.
Note: No afiliation whatsoever to Mactracker, I find the app to be super helpful and use it often.