I see an altar as a magical workbench, it's a place to gather together whatever bits and pieces you're using for a particular working; candles, incense, herbs, etc. You set it up, perform the work and when it's finished take everything off again.
A shrine is a place of devotional focus to a deity(ies) or spirit and is a permanent sacred space. It usually has things that are associated with whatever deity you're honouring, including images, flowers, stones, candles and is a place to leave any offerings. I've included an image of my Dionysos shrine on my pictures as an example.