ASA, which stands for acrylic styrene-acrylonitrile, is an amorphous, opaque thermoplastic that is structurally similar to ABS.
In many cases, they are quite interchangeable in purpose but ASA has a few key differentiating features that make it suitable for certain applications over ABS. ASA is significantly more resistant to UV light and weathering, making it the better choice for applications with extended outdoor exposure. ASA can boast slightly better mechanical properties but it can also cost slightly more in some cases.
Since ABS is so popular and has many more options, it is generally recommended to go with ABS unless outdoor exposure is a concern.