Learn English – a sense or senses

Now coming to the question - Recently i looked up in the internet as to: when is a definite article used ? It says - "We use the definite article in front of a noun when we believe 'the reader' knows exactly what we are referring to."

I think this statement is a general statement about the use of definite article. My first question, Am i right to say that this is a general statement ?

Yes, you are right. "The" can serve a number of specific grammatical functions that are not covered by this general statement. For instance, in the phrase "the more, the merrier", the matching instances of "the" have nothing to do with the reader's knowledge about what is being discussed. They're simply part of a fixed structure.

Second question is, What if we use 'readers' in place of 'the reader', will there be a change in meaning then ?

There will be little or no change in meaning, but there will be a slight change in style. Using the plural is the unmarked way to make general statements. Using the singular can be more vivid, and also more literary, which may or may not be what you want. It also has an air of instruction to it, indicating that the speaker/writer is about to embark on a discussion that singles out a specific representative of each class being discussed. In cases like yours, it can, but need not, indicate a one-to-one relationship. Sometimes the plural is used to avoid the necessity of a singular third-person pronoun, which would have to be gender-marked ("he" or "she", "his" or "her", etc.) or inanimate ("it", "its", etc.) or enumerate all genders ("he or she", "his or her", etc.). For example, compare:

"Banks are financial institutions where a lender meets a borrower. The lender gives {his, his or her} money to the borrower..."

with this:

"Banks are financial institutions where lenders meet borrowers. Lenders give their money to the borrowers..."

Both have weaknesses and both strengths. The former is more specific: one lender gives money to one borrower. (This may or may not match reality, but if it is what you wanted to convey, it would be more accurate.) However, the latter is concise and avoids the games writers need to play in order to avoid excluding members of a gender.

In sentence (1), 'a lender' expresses just one lender or any lender ? In sentence (2), 'lenders' expresses more than one lender or any lender in general ?

In sentence (1), the context and the reader's prior knowledge make it clear that you are talking about any lender. In sentence (2), the context and the reader's prior knowledge make it clear that you are talking about any lender in general. If the writer suspects that context and the reader's prior knowledge are insufficient to make things clear, and clarity on this point is important, he or she should choose another means to clarify things. For instance, if you wanted to clarify that the relationship between a lender and a borrower is one-to-many (a lender may lend to multiple borrowers, but a borrower only borrows from one lender), you might say:

"Banks are financial institutions where a lender finds one or more borrowers..."