Memory = (1) something that you remember and see as a picture in your mind, especially an experience that you had a long time ago: ‘The old teddy bear brought back memories of my childhood.’ (2) the power of retaining and recalling past experience: ‘He had a good memory when he was younger.’
Souvenir = something that you buy or obtain in a particular place (especially while on holiday) and keep to remind you of it: ‘The vase is a souvenir of my week in Venice.’
Memorize = learn something so that you are able to repeat it exactly; learn by heart: ‘Have you memorized your lines for the play?’
In memory of sb, to the memory of sb or as a memorial (to sb) = as a permanent reminder of someone after they have died: ‘The charity was set up in memory of the late Lord Hinton.’ ‘It was built as a memorial to all those who died in the war.’