What is a smash cake???

I make them for 1st birthdays. You make you regular cake for the guests and then you make a much smaller one (I make a 6") for the birthday boy/girl to dive into with their tiny hands and face. Everybody loves that picture of the birthday baby with cake-covered hands and face.

Are smash cakes only for a baby's 1st b-day? Is it ever used in other events?

I've seen them for other than one year birthdays, but actually seems like you could have one for almost anything...like somebody wanted a divorce cake, a smash cake with the ex's face or name might be fun!

I've only ever seen them (and done them) for first birthdays.
Tough for a princess birthday this year, I did min-princesses for the 3 girls at the party, instead of the cake. They LOVED having their own cake.