The Old Spanish Trail was an overland pack-train trade route rather than a pioneer migration trail between Santa Fe, New Mexico and Los Angeles California from 1829 to the mid-1850s. Portions of the trail were explored as early as 1776 but left mostly unused. In 1829-1830 the Santa Fe merchant-explorer Anotoni Armijo combined information from several previous explorations and led a group of 60 men and 100 pack animals from Santa Fe to Los Angeles. Indian goods like woolen blankets and basketry were traded for California horses and mules which could be sold in Santa Fe, or (via the Santa Fe Trail) in Missouri. Later in the history of the trail an extensive Indian slave trade developed which had a significant impact on Indian tribes along the route.
Part of the reason the Old Spanish Trail was used was because it linked via the Santa Fe Trail to Missouri and the United States, and via the the Camino Real to Chihuahua and Mexico City in Mexico. 
There were very few European settlements on this rugged route between Santa Fe and Los Angels except near each end of the trail. The Old Spanish Trail had several additional branches which are not shown in the nearby map.
The opening of easier wagon-friendly routes to the north (Pony Express or Overland Trail) and south (Butterfield Overland Mail) resulted in few Europeans settling along this difficult pack-animal route. However, because of the slave trade along trail the Indian tribes were cautious, hostile, and themselves engaged in slave trading and raids.
Many of the settlers along the trail in present-day southern Utah, Nevada, and California arrived via the Mormon Trail and the Mormon Trail to Southern California which overlapped the Old Spanish Trail from about Cedar City, Utah to Los Angeles California.