广告 = advertisment 宣传 = to disseminate, to announce (in a formal way), to broadcast

宣传can be a noun or a verb, 广告is a noun only. In Chinese people's mind, maybe the most noticeable difference between them (as nouns) is their collocations. 广告 is usually used in commercial context. 给产品做广告/打广告（打广告 is somewhat colloquial) On the contrary, 宣传 is not usually in commercial context. And it is more used with abstract things.

宣传can be used to introduce some famous people or activities and so on.广告used to introduce the products in order to sell them.

Hi Hannah, I'm no Chinese teacher but as a native speaker what I can say is: the English equivalent to 宣传 is "promotion" or "branding", and that to 广告 is "advertisement". therefore, 广告 is a form of 宣传, other forms can include for example fan events, sponsorship in charity & etc. In short, 宣传 is larger than 广告 and 广告 is a form of 宣传. Hope this helps. :-)