I was wondering whether this is the right forum to ask a management question that is related to marketing

Apologies if it is not

I would llike to understand the difference between product development (ansoff product-market grid says new products targeted at existing market)  and Horizontal diversification (kotler text deems it as  new products sold to existing customer groups)

Are they the same concept or am i missing soemthing here like the concept of new products or the difference between existing markets and existing customer groups?