What's the difference between semaphore and electric signals?

There is no functional difference between the two. By default the game will construct semaphores before a certain date defined in the Advanced Game Options > Construction section, to mimic the introduction of electrical signals in real life. Holding down Ctrl overrides that and builds semaphores instead.

The reason why everyone's screenshots show electric signals is that the default preferences setting is to use semaphores before 1950 and electric signals after, while the default game start date is also 1950.

There's no functional difference between the two.