A descriptive grammar analyses the language people use and synthesises rules from the patterns of usage. A prescriptive grammar (aka traditional/standard) approaches it from the opposite direction- it has a body of rules and tells people to apply these rules to the language they use.
A prescriptive grammar could tell you that 'If I was' is wrong because we should use the subjunctive form there and say 'If I were', while a descriptive grammar would say that both forms are used so both forms are acceptable. 'Textbook grammar' is not a specific term but can be used to refer in a negative way to the grammar rules given in textbooks, because they are oversimplified, outdated or inaccurate.
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