Any wire that carries a current will have a magnetic field created around it. For a long straight wire, the field encircles the wire.

If a second wire is next to this wire --- in the magnetic field of the first wire --- and it carries a current, there will be a force on the second wire proportional to the current it carries and the strength of the magnetic field of the first wire.

Force on wire 2 = (current in wire 2)(Length of wire 2) x (Magnetic field of wire 1)
Force on wire 1 = (current in wire 1)(Length of wire 1) x (Magnetic field of wire 2)

As a cross product, the direction of the forces will be such that the wires are ATTRACTED to each other if the currents are in the same direction and REPELLED if the currents are in opposite directions.

You can remember this from a demonstration with a coiled loop of aluminum wire . When a current of 30 A is passed through the wire, the coil contracts and jumps up. When the coil leaves the puddle of mercury at the bottom, the current is cut, the magnetic fields disappear and the coil relaxes. Then it touches the mercury, current flows again and the coil contracts and jumps.