What is the difference between “group” and “band”?

What is the difference between group and band when applied to assemblages of musicians who play music together?

According to Wikipedia,

A musical ensemble, also known as a music group or musical group, is a group of people who perform instrumental and/or vocal music, with the ensemble typically known by a distinct name.

For example, ABBA is described as being a pop group by the Daily Telegraph but as a pop band elsewhere on the Internet.

There's no absolute difference.

Band originally had closer associations with larger assemblages of musicians (Brass Band, Big Band, etc.). When smaller ensembles became more practical because of new music technology (amplifiers, electric guitars, etc.) mainstream parlance tended to use the term group rather than band.

Group has always remained a bit more 'mainstream'; it tends to get used more by people outside the music industry, and those who aren't so interested in the music itself. For example, a TV news item is more likely to refer to U2 as a Group.

Band remains common within the music industry itself, and among devotees of the product. People who listen to U2 are more likely to call them a Band.

Musical ensembles working with many specific 'genres' are commonly referred to as bands even in mainstream parlance. For example, this graph shows that Jazz Band is far more common than Jazz Group. But that doesn't apply to all genres; as this graph shows, Pop Group outweighs Pop Band by a considerable margin.

In short, Band today has a little more 'street cred' than Group, even though the actual meaning isn't really any different.

I'm assuming this question is focused on the meaning of the words in contemporary popular music -- not the meaning from other eras.

I think you can use the term group to refer to a band, but the term band does not apply to all groups.

A band strongly implies that the members play instruments, aside from just singing and dancing. The Beatles and U2 are examples of bands.

A group can be any band. The Beatles and U2 are also groups. But a group can also refer to performers who don't play instruments onstage (or in a music video). These performers primarily sing and/or dance. Usually "session musicians" or a "backup band" perform the instruments to the side of the stage. Sometimes, the music is completely or mostly pre-recorded (with just a live DJ). For example, it's rare to hear the term "hip hop band." Instead, you would say "hip hop group," because the performers that are center stage are using only vocals and dance.

One notable exception: There is the idiom boy band. Wikipedia describes it as such:

A boy band (or boyband) is loosely defined as a vocal group consisting of young male singers, usually in their teenage years or in their twenties at the time of formation,[1] singing love songs marketed towards young females. Being vocal groups, most boy band members do not play musical instruments, either in recording sessions or on stage, making the term something of a misnomer. However, exceptions do exist. Many boy bands dance as well as sing, usually giving highly choreographed performances.

Full text is https://en.wikipedia.org/wiki/Boy_band. Throughout this page, group is consistently used to refer to vocalists, and band is consistently used to refer to instrumentalists.

Examples of boy bands are Backstreet Boys and 98 Degrees. I believe the origin of the boy band goes back to The Monkees in the late 1960s. Boy bands are typically put together by record companies and are given hits to perform. In such cases, their success is pretty much guaranteed, because the record labels have access to the best producers. Sometimes they have skills as musicians (guitar, keyboard, etcetera), but more often than not they are chosen for their vocal skills and appearance.

The female counterpart is called a girl group. Again, these singers focus on vocal and dance performance. Salt 'n' Pepa and Wilson Philips are a couple of examples. Girl groups dominate the pop charts in Asia. A particularly funny one is the Akihabara 48 in Japan, where 100 young ladies constantly compete to be a part of the 48 you might see onstage on any given night.

One can't help but think that the term "boy band" became an idiom because of its alliterative quality.

band::group as Lyft(r)::Uber(r)

In my own experience as a musician, having performed in and attended concerts for both "bands" and "groups" of differing genres myself, there seems to be a connotation that the mebership of a 'Band' typically considers themselves (or is marketed as) very closely-knit, more intimately and/or emotionally bonded, or otherwise enjoy a more casual relationship with one another. Consider the non-musical use of 'band' in reference to Robin Hood's 'Band' of Merry Men. Whereas a 'Group' connotes a more distinguished, still relatively closely-knit, yet more professional relationship between the members. Though members of a 'group' may still hold fondness and amiability toward each other, the branding tends to focus more on their combined proficiency rather than the members' relationship to each other.
Consider the Pat Methany Group whose jazz-fusion audio imagery is mind-blowing. The two founding members particularly enjoy (or may have enjoyed) a long-lived friendship, albeit they market the musical prowess of the group itself with more distinction. Now compare that to a Boy 'Band,' like One Direction or N'Sync, whose center of marketing revolves around them all being the best of buddies who grew up together and do nothing more than hang out together all the time experiencing young life as an inseparable team. Truly the fans love them for who the members are and, oh by-the-way, perhaps the sound really good! Something to Ponder since I saw Ringo in the thread: Can you define, in their vast career, when the Beatles transitioned from a Group to a Band (and/or Vice Versa)?

A group performs music with mainly vocals or only vocals. Example backstreet boys and spice girls.

A band performs music with mainly vocals and instruments. Maroon 5 and Cold Play.

Nowadays the two terms are interchangeable, but their origin is different. Band as a noun is more idiomatic and related to music, while th meaning of group in the musical sense developed much later:

Band:

• The extension to "group of musicians" is c. 1660, originally musicians attached to a regiment of the army.

Group:

Meaning "pop music combo" is from 1958.

(Etymonline)