Dynamics GP tables are divided first by module. All the sales module tables start with SOP. All the receivables tables start with RM. All the payables tables start with PM... you get the idea. So if you're writing a query you can quickly eliminate almost all of the tables and just settle in on 30 or 40.
Go to the Knowledge Base Table Reference on this site, and review all the tables in the module that you want to learn. By far, SOP (Sales Order Processing) is the most popular request so I'll start there. (but it's closely linked to RM, the customer master table is RM00101)
http://dyndeveloper.com/DynTable.aspx?ModuleID=SOP
Further, the tables are divided by usage. Tables that start with 00 are setup tables. Tables that start with 40 are setup tables. There's also the concept of 'work', 'open', or 'history'; although not all the modules use all 3 of those concepts. Tables that start with 10 are open tables, tables that start with 20 are open tables and tables that start with 30 are history. SOP only uses the 10 and 30 tables. PM uses all 3. Welcome to my world.
The concept is that when a user is typing a document it is in work; when they save it it is in open and when we're done with it it goes to history. The movement from work to open to history is different in every module. Sorry about that.
As an example in SOP the SOP10100 table is sales open and the SOP30200 table is this exact same table but in history.
Last... no all tables follow this schema. <smiles> but most do.
That's a good enough start for now. Find a module, find a query, and ask a question here. We'll help.