Can refer to a variety of ships, but typically refers to the Tech 1 industrial ships. They are slow and relatively defenseless, but have large cargo holds, which make them ideal for transporting loot/minerals around.
Tech 1 industrials have a decent amount of cargohold but are prone to ganking since they are slow and relatively defenseless. Flying these in low/null sec makes one a priority target; suicide ganking is also common in highsec aimed at pilots carrying expensive loot within their cargoholds.
Tech 2 Industrials serve specialized roles. Deep Space Transports (DSTs) are slow but have good tank and +2 warp core strength. Blockade Runners have reduced cargo carrying capacity but are faster and can fit a covert ops cloaking device on top of being immune to cargo scanners.