The free version needs no account and has no limits: the 2D map, the full 3D sky over a flat horizon, place search and every timing the app calculates.
Pro adds photorealistic 3D terrain, satellite imagery, faster place search and the astro events calendar. It is sold in blocks of three, six or twelve months. One payment, nothing renews automatically, and when it runs out you keep the free version, your account and your saved views.