Just as the Lincoln Memorial pays tribute to Abraham Lincoln, a memorial is anything that symbolizes or celebrates someone who died. Every gravestone in a cemetery is actually a small memorial.
A memorial is a service or statue dedicated to someone who died. A memorial service celebrates the life of the deceased. Something written about the dead person can also be called a memorial, and you could say donating to a dead friend's favorite cause is a memorial to them. If it honors the dead, the word memorial probably fits.