From Quantum Circuits, there are two statements that are not clear.
Quantum Circuits Quantum circuits are collections of quantum gates interconnected by quantum wires. The actual structure of a quantum circuit, the number and the types of gates, as well as the interconnection scheme are dictated by the unitary transformation, U, carried out by the circuit. Though in our description of quantum circuits we use the concepts input and output registers of qubits, we should be aware that physically, the input and the output of a quantum circuit are not separated as their classical counterparts are; this convention allows us to describe the effect of unitary transformation carried out by the circuit in a more coherent fashion. In all descriptions of quantum circuits in addition to gates, we see quantum wires that move qubits and allow us to compose more complex circuits from simpler ones that, in turn, are composed of quantum gates. We compose components by connecting the output of one to the input of another; we also compose operations when the results of an operation are used as input to another. The composition does not affect the quantum states. The quantum wires do not perform any transformations in a computational sense; sometimes we can view them as transformations carried out by the Pauli identity operator ∑I.
"the output of a quantum circuit are not separated as their classical counterparts are; " mean? What does it mean are not separated as their classical counterparts? In classical, we have transistors acting as gates - so aren't those connected too?
"we see quantum wires that move qubits and allow us to compose more complex circuits from simpler ones that, in turn, are composed of quantum gates" mean. What are these wires physically in a quantum computer? I understand that we read quantum circuits from left to right and that they are unitary transformations. Are the wires just a way to show this flow is from left-to-right?