What kind of Instrument is this?

88 keys stand in a row,
Black and white are they.
One thing that's great to know,
Is each one sounds in a quite different way.

True, all make music together,
When played correctly and in proper order.
But one thing is that they work as a team,
Not separate, but united, they sound as a dream.

At one end, a key is high.
Down yonder, another is low.
When put with each other,
Well, music's made & I think you know why.

Don't get stressed as to what it is
I'm sure you really know.
This ain't exactly a very hard quiz,
You just gotta keep guessing to and fro?