The prayer offered just after sunset is Maghrib. Its name comes from the Arabic word "gharb" (west/setting), referring to the setting of the sun. Maghrib is the fourth of the five daily obligatory prayers and consists of three units (rak'at). Its time begins immediately after the sun sets below the horizon and lasts until the red twilight (shafaq) disappears from the sky.
The other prayers are performed at different times of day:
Fajr is the dawn prayer, performed before sunrise.
Asr is the afternoon prayer, performed in the mid-to-late afternoon when the shadow of an object equals its length (or double its length, according to some schools).
Isha is the night prayer, performed after the twilight has completely disappeared, which is later than sunset - not immediately after it.
Maghrib has the shortest window of any of the five prayers. Because it must be prayed promptly after sunset, the Prophet (peace be upon him) advised Muslims not to delay it.