What's the difference between semaphore and electric signals?

In OpenTTD you can use electric signals and semaphore signals. In every instance I've ever seen people use electric signals to direct their trains. On the wiki the only mention of semaphores is "don't hold down control otherwise you'll end up with a semaphore". This suggests to me that semaphores are unwanted and have a different function to normal signals that is only applicable in complex circumstances. I couldn't find anything on the internet detailing their function.

What is the functional 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.