There are 12,000 species of ferns in the world, but I can only identify about five of them without looking them up. Christmas fern (Polystichum acrostichoides) is one of the easiest.
But you can easily identify Christmas fern at other times of year, too. Look at the shape of the leaflets: they're bent into a J shape near where the leaflet joins the stem. Think of them as little Christmas stockings, and you will always recognize the Christmas fern!
In the wild: Christmas fern is common in our local woods, especially along streams and on hillsides. Its green should stand out at this time of year.
In your yard: These are a great option to get a little year-round green in a shady spot that doesn't get too dry. Christmas ferns are widely available at local nurseries, but they aren't cheap -- ferns are hard to propagate. They can be divided if you've got a friend with a healthy clump: do it in the spring when they're sending up new shoots, and make sure you get several fiddleheads per division.