This is a standardised way of representing an electrochemical cell, making the contents of the two half-cells clear.

The half-cell with the more negative electrode potential is written on the left. If it consists of a solid metal and its ions in solution, e.g. Cu and Cu2+, it is written like this:

Cu | Cu2+

The | symbol represents the boundary between the solid and the solution.

If both species are in solution they are written next to a platinum wire, Pt |, with the more reduced species placed next to the platinum and separated from the other by a comma:

Pt | 2Br-, Br2

If one of the species in solution actually has two components, e.g. O2 and 2H2O, they are written in square brackets:

Pt | 4OH-, [O2 + 2H2O]

The other half-cell is written in the same way on the right, and the two are separated by the symbol ¦¦, which represents the salt bridge. Thus a complete cell diagram will look like this:

Cu | Cu2+ ¦¦ [O2 + 2H2O], 4OH- | Pt

This cell consists of water and oxygen oxidising copper to copper(II) ions, and reducing themselves to hydroxyl ions in the process.