GMATPrep™ is CAT (Computer Adaptive Test), simulation software which has similar user interface as real GMAT exam. It can be downloaded from mba.com
This is very subjective question and answer varies. Three months are sufficient for an average student.
Of course OG-11; without a doubt.
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Get only OG-11th Edition now.
They are good as extra material for preparation, but should not buy them as the primary material. Use latest editions instead.
No. You have to buy them, if you need to.
Official Guide Review is (/are) the book (/books) published by the makers of the GMAT. OG consists of a lot of practice questions. All questions are real but retired, which means the questions in OG were actual GMAT once. The actual questions give us idea of how questions are asked and what are the topics covered in real GMAT. Thus OG reviews have highest credibility.
What are the most common abbreviations or short-forms used in GMAT studies or in such online forums that I should know?
MBA: Master of Business Administration
b-school: business school.
GMAT: Graduate Management Admission Test
GMAC: Graduate Management Admission Council
AWA: Analytical Writing Assessment. It’s the first section of the test, where you’re asked to write two essays.
q: quant/ quantitative (one of the sections of GMAT where the overall math skills are tested.)
v: verbal (One of the sections of GMAT where the overall English skills are tested.)
PS: Problem Solving (Type of questions, part of quant section)
DS: Data Sufficiency (Type of questions, part of quant section)
SC: Sentence Correction (Type of questions, part of verbal section)
CR: Critical Reasoning (Type of questions, part of verbal section)
RC: Reading Comprehension (Type of questions, part of verbal section)
OA: Original answer/ official answer. (not necessarily the correct answer though. 
)
610 m48v26: It means that a student has scored 610 in GMAT with 48 points in Quantitative/Math section & 26 in Verbal/English section.
splits: the word used in forum lingo splits is the detailed score where you provide marks obtain in verbal & quant… in the above score 48Q/26V is the split.
GMATPrep™: It’s simulation software, CAT, used for practice GMAT
CAT: Computer Adaptive Test. (Many GMAT coaching companies provide CATs.)
OG-11: Official Guide Review 11th Edition, a GMAT book.
OG: Simply Official guide reviews, (There have been so many versions of OG so far.)
PR: Princeton Review, one of the GMAT preparation companies
MGMAT: Manhattan GMAT, one of the GMAT preparation companies.