Where it came from
Electrum's own site states that it "was created by Thomas Voegtlin in November 2011". The repository backs that up: the earliest commits on the master branch date to December 2011. It is one of the oldest pieces of Bitcoin software still in active use.
The site also names the entity behind it: "Electrum Technologies was founded by Thomas Voegtlin in 2013", with an address in 10999 Berlin, Germany. The software is MIT-licensed and, in the site's own words, "distributed under the MIT licence by Electrum Technologies GmbH". That is the reason this entry records a German limited company rather than Undocumented, even though the code itself is an open project with many contributors.
What it built
The problem Electrum solved in 2011 was that running a wallet meant downloading the whole block chain, which then took days. Electrum instead uses simplified payment verification: it downloads block headers, gets its transaction history from an indexing server, and checks each transaction against the headers. The server learns which addresses you asked about, which is the tradeoff, and the answer to that tradeoff is to run your own server. Two implementations exist for that: ElectrumX in Python and Electrs in Rust.
Electrum also popularised recovering a wallet from a phrase of ordinary words, in its own seed format, which predates BIP39 and is now how nearly every wallet handles backup. Lightning support arrived later.
Who maintains it
Electrum's release process names its builders openly, and several of them are pseudonyms. The site records that the current executables "have been signed by ThomasV, SomberNight, Emzy, felixb_f321x, svanstaa", and that changes to the website itself "must be signed by both ThomasV and SomberNight". SomberNight's release keys are published in the repository under that name. This directory lists those handles exactly as the project publishes them and makes no attempt to go behind any of them.
What it changed
Two things outlast the code. The first is the Electrum server protocol, which became the common interface a great many other wallets speak, so one server can back several wallets. The second is that a wallet can be lightweight without being custodial, which is the distinction a lot of later software was built on. Listing here is not endorsement.