通知 (tōngzhī) is the everyday word for 'to notify' or 'a notice' — it works as both a verb and a noun. 告诉 (gàosu) is only a verb and means 'to tell' in a personal, conversational way, while 通知 is more official (school, company, government). For written announcements, 通知 is a general notice; 公告 (gōnggào) is a formal public announcement, and 启事 (qǐshì) is a classified ad or posted notice.