Schlager is the German word for "Hit", initially it has the same meaning, a big success.
Later "Schlager" became the term for a kind of easy listening pop music, trying to please most people as possible in German speaking countries and Benelux.
In "Schlager" nearly all kinds of music styles are possible. Songs from the musical genres Country, Rock'n'Roll, Pop, Swing (and in few cases Hard Rock) counts as Schlager, when the English lyrics are exchanged by German or Dutch. Also in the other direction, where a Schlager became with English lyrics a pop song. Would the Band ABBA sung their songs in German, this music would count as Schlager.
Much Schlagers are from USA, UK, other European countries, especially Netherlands, in some cases the German version is more popular, than the original.
By the way, German Schlagers were exported, much of the Dutch Schlagers are German Schlagers with Dutch Lyrics. Few German Schlagers became successfull in the USA.
Some Schlager songs are sentimental. Some Schlager songs refers to German folk and German drinking songs. Some Schlager songs are arrangements of classical music.
Schlager songs were very popular from the 1920's till the 1970's. In this time, songs in German language almost counts as Schlager. This changed in the 1980's, where the "New Wave" reached Germany and much bands began to sing in German, this was called "Neue Deutsche Welle" and was beside singer/songwriters the first German music without the "Schlager" label. Germany's answer to "New Wave" lasts few years, but was door opener and encouraging much bands to sing in German. And today it's common, that Germany has much different music in German language that isn't seen as Schlager.
In East Germany it was different. There was the rule "When you want to sing on a record, you have to sing German", so in East Germany was a German pop music beside Schlager, long before West-Germany discovered this way.
Much Schlagers of today are in the style of electronic Dance music "Disco Fox".
Arrangement and orchestration: Many things are possible, from a small band with drums, bass and two guitars to a big orchestra with brass and strings. Most songs are sung, most of them by one main singer, some with two main singers, often some accompaniment by a choir and in few cases a choir as the main singers. In the 1980's they began to use electronic instruments and sequencers more and more frequently.
In the 1970's, there were TV shows and sampler records containing both, German Schlager and English Pop music, this was possible and worked. The famous German TV show "hitparade im ZDF" showed the most successful pop songs in German language, till the coming of "Neue Deutsche Welle" this were Schlagers. It was a show, where the vocal performance had to be live, accompanied by tapes with the orchestral backing from the original recording. Later they tried to open the show to other music from Germany in English language and also the use of full playback, so the vocal performance was prerecorded. Few years later, they turned back to semi playback and German language. But in the 1990s Schlager wasn't so much popular anymore, so the show "hitparade" wasn't continued.
At least, Schlager was a collective term for different kinds of pop music with German lyrics.