Conduit: a pipe, tube, or enclosed channel for conveying fluids or gasses; and when used in electrcial work, a tube (round or square) or a trough (usually square; metal or plastic; fully enclosed or open on one side) which may contain one or more separate ducts, and is used to contain and protect one or more insulated electrical wires. Images of conduits: https://www.google.ca/search?q=conduit&biw;=1366&bih;=655&source;=lnms&tbm;=isch&sa;=X&ved;=0ahUKEwiTm-SO1pPKAhWHrB4KHekWBjQQ_AUIBigB Duct: a channel (enclosed; square or rectangular) or a tube (enclosed; round) used to convey a liquid, gas or air; also a naturally occuring channel in the ground, usually containing water. duct image: https://www.google.ca/search?q=duct&biw;=1366&bih;=655&source;=lnms&tbm;=isch&sa;=X&ved;=0ahUKEwiskdHG1pPKAhXLGx4KHXdTAWIQ_AUIBygB NOTE: If you cannot see the images (because they use Google web browser), then use your own web browser and type the search terms "duct" or "conduit".

They are similar in that they both transport or transmit things directly. In fact, they have the same etymological root (Latin : ducere - to lead). The difference in practice is in conventional usage and a lot of the usages will be jargon related to building. If you have specific examples, perhaps we could help further.