Semi-regular verbs are those that undergo a "stem change" in some tenses, but they maintain the regular endings. For example: empezar=to start -present tense (yo empiezo, tú empiezas, él empieza...). However totally irregular verbs are those that ungergo a "complete change" both in the stem and the endings. For example: ir=to go -preterite (yo fui, tú fuiste, él fue...).
I hope this made it a bit clearer.