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Logic gates
[A logic gate consists in] three connections where there may or may not be some electricity. Two of those connections are places where electricity may be put into the device, and the third connection is a place where electricity may come out of the device. (Scott, 2009 p.21)
Logic gates are the basic building blocks of digital computing. A logic gate is an electrical circuit that has one or more than one input and only one output. The input controls the output and the logic determining which types of input (on/off) lead to specific outputs (on/off) is identical to the truth-conditions of the Boolean connectives specifiable in terms of truth-tables. Physically, what 'travels through' the gates is electrical current and what constitutes the 'gate' is a transistor responding to the current. At the next level of abstraction it is bits that go into the gate and bits which come out: binary information that may be either 1 or 0.
References
Scott, J. Clark. 2009. But how do it know?: the basic principles of computers for everyone. Self-published.