Spanish verbs are one of the most complex areas of Spanish grammar. The Spanish verb system is separated into 8 distinct tenses, referring to both time and mood.
Spanish verbs are conjugated in four categories known as moods: indicative, subjunctive, imperative and non-personal forms, which contains the three non-finite forms that every verb has: an infinitive, a gerund, and a past participle. This past participle can agree for number and gender, giving it four possible forms and be considered to an adjective derived from the verb.
Read more about Spanish Verb Forms. [Source]