The best explanation I can find-

I found this article on Wikipedia (I know...wikipedia isn't reliable, but trust me...this is the best explanation of Semana Santa. I posted twice today, so make sure you read my second post "I bleed Spanish blood"

Seville, Spain

A large paso paraded during Holy Week

Seville arguably holds some of the most elaborate processions for Holy Week. A tradition that dates from Counterreformation times, or even earlier. The "Semana Santa en Sevilla" is notable for featuring the procession of "pasos", lifelike wood sculptures of individual scenes of the events that happened between Jesus' entry in Jerusalem and his burial, or images of the Virgin Mary showing restained grief for the torture and killing of her son. Some of the images are of great antiquity and artistic masterworks. These "pasos" (which usually weigh over a metric tonne) are physically carried on the neck of costaleros (literally "sack men", for its distinctive -and functional- headdress). The "costaleros" (from 24 to 48) are hidden inside the platform of the "paso", so it seems to walk alone. Historically dock workers were hired to carry the "pasos". From 1973 onward, that task has been universally taken over by the members of the confraternities who organize each procession.

The "pasos" are set up and maintained by hermandades and cofradías, religious brotherhoods, confraternities or sodalities, which precede the "pasos" (up to 3) dressed in penitential robes. Some of the processions are near 3000 persons each. In Seville, but for some officials, "costaleros", acolytes and a few other exceptions every participant must wear penitential robes and be hooded. A brass band may accompany the group, playing funereal religious hymns or "marchas" written for the occasion. Some processions are silent with no musical accompaniment. As each procession leaves its home church, called a salida, as well at its return (the entrada) and along the march route there are special extemporaneous songs offered by individuals in the crowd or a balcony. This songs are generically called saetas (arrows).

A total of 58 processions (as of 2007) occur during the week, from Palm Sunday through Easter Sunday morning. On Maundy Thursday there are two sets of processions. One group occurs during the afternoon and evening. The second set begins near midnight and continues until early afternoon on Good Friday. The most famous processions occur this night (the madrugá), including Jesus del Gran Poder, Esperanza - Macarena, and Esperanza - Triana.