Pretty new to Apex in general, and completely new to Batch Apex, but that's what I'll need to use in order to do the job before me so I'm tyring to get my head around the differences. The Tralihead module on Async Apex has a good example, but doesn't answer my question. The developer guide (https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_batch_interface.htm) shows a couple of different examples, but still doesn't address the quesiton.

So in its shortest form, my quesiotn is what's the difference between these two class declarations? Besides the obvious I mean.

public class SearchAndReplace implements Database.Batchable<sObject>{}

and 

public class batchClass implements Database.batchable{}

One specifiec <sObject> in the protitype, the other doesn't. That suggests it's optional, but I've yet to find any docuemntation that outlines when to use it and when not to.

The difference in the classes themselves is mostly in the start method -- the first returns a QueryLocator and the second an Iterator (the other difference being that I mostly understand what the first one is doing). But it doesn't explain why the <sObject> exists in one prototype and not the other. There may be a use case where you'd use Batch Apex for something besides processing large numebrs of database records, but neiter of these examples does that -- they just differ in how (and presumably how many) they process the records in question.

That's not the only question these examples raise, but trying to tsneak in another one woudl be cheating. So I'll start with this one and see if it helps with the others.