Shounen Ai and Yaoi are both genres in manga that depict romantic relationships between men, but they differ primarily in their level of explicitness and focus.
Shounen Ai, also known as Boys' Love or BL, emphasizes emotional aspects and the development of a romantic relationship, typically being more gentle and subtle without explicit sexual content. On the other hand, Yaoi is generally more sexually explicit and can be much more graphic in its depiction of physical relationships between male characters.