A smuggler was anyone who sold or transported contraban or controlled substances in the Imperium.
Smuggling on Arrakis
During Harkonnen rule on Arrakis, smugglers tended to remain covert for fear of retribution. However, when House Atreides took over the Arrakis fief and all spice mining on that planet, Duke Leto Atreides attempted to establish detente with the smugglers by informing them that he would ignore their operations, on the provision that they forwarded to Leto a percentage of their profits. This worked to some degree, as some of the smugglers assisted in capturing Harkonnen agents left behind from the previous fief, and turned them over to Atreides authorities.
Esmar Tuek was a known spice smuggler during the time when House Atreides took over spice mining on Arrakis in 10,191 A.G. When House Atreides was attacked by House Harkonnen in 10,191AG, some of Duke Leto's men managed to escape the ensuing onslaught by allying themselves with smugglers. The hope was that by remaining on Arrakis among the organised underworld, they would one day be able to exact retribution for the way in which their beloved Duke was defeated. Of particular note among the Duke's men to do this were Gurney Halleck, the Atreides Warmaster, and several men under his direct command.
Many years after returning to Caladan, Halleck once more went to his smuggler friends on Arrakis to obtain information.