Smuggling is the clandestine shipment of illegal goods, or even clandestine shipment of legal goods to prevent taxation or some other impediment.
Smuggling thrived under the Galactic Empire, for there were many items which the Empire wished to control for various reasons. For instance, spice: the Empire didn't necessarily care about the physiological repercussions that spice might cause to its citizens, but they wished to control the traffic of spice so that they might benefit from it financially (and also so that their opponents would not benefit from it).
Another example is gunrunning: the Empire wanted to control weapons availability in order to prevent criminals, and even more so, insurgents, such as the Rebel Alliance, from being well-armed.
Smugglers can make a decent living - and in some cases command great financial success if they are good enough at what they do. On the MUSH smuggling characters are often employed by one of the various criminal orgs either on a job to job basis or under a long-term arrangment. Some criminal organizations, such as Black Sun, and Maffi / Zhao are difficult to sever ties with however once a deep relationship is formed.
Often seemingly legitimate shipping and frieght companies enjoy a lucrative, under the table smuggling operation. Their legitimate front affords them the luxury of eased security and customs searched aboard their ships.