DDS is DirectDraw Surface, the texture format games ship. Its point is that the GPU reads the compressed blocks directly, so BC1/BC3 textures never need unpacking at runtime. FileSorcery reads it and converts out; writing good block compression is a job for a dedicated compressor.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a DDS file above and choose afterwards.