Two quantum systems are said to be entangled when a measurement on one gives information about the other. Usually this happens when the two systems have interacted in the past in some way. A common example is the entanglement of two electrons. Electrons have indeed a property called spin, which can assume value 1/2 (spin up) or -1/2 (spind down). Two electrons in the same position needs to have opposing spins, due to Pauli exclusion principle. So if you have two electrons in the same place, that are then separated, you get an entangled state. A measurement of the spin of one particle, immediately gives you the spin of the other.
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