The present perfect tense is used to describe actions that began in the past and continue in the present or have relevance in the present, without specifying a definite time period. In contrast, the simple past tense describes actions that were completed entirely in the past. Some key differences are that the present perfect can be used with expressions like "ever" and "never" while the simple past requires specific time references like "yesterday" or "last year." Examples are provided to illustrate using each tense to talk about completed past actions versus ongoing/relevant present actions.