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In this review, a pedagogical introduction to the concepts of slow-roll inflationary universe and number of <i>e</i>-folds is provided. In particular, the differences between the basic notion of number of <i>e</i>-folds (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>N</mi><mi>e</mi></msub></semantics></math></inline-formula>), total number of <i>e</i>-folds (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mi>N</mi><mi>T</mi></msub></semantics></math></inline-formula>) and number of e-folds before the end of inflation (N) are outlined. The proper application of the number of <i>e</i>-folds before the end of inflation is discussed both as a time-like variable for the scalar field evolution and as a key parameter for computing inflationary predictions.