Here's a pithy explanation from the Guardian:
Scots, used predominantly in the Lowlands, Grampian, and populous central belt of Scotland, is commonly considered to be a language in its own right, sharing the same Old English ancestry as modern standard English, though having developed separately. It even has its own dictionary. ...
Scots has its own esteemed body of literature, most notably the poems of Robert Burns, and is peppered with words that leave most other English speakers scratching their heids (another Scots dialectal form): shuggle for shake, niffle-naffle for wasting time, the exuberantly satisfying gontrum niddles for a cry of joy, and countless others. Swithering, for hesitating, may have become more familiar during the independence referendum campaign as applied, not entirely flatteringly, to undecided voters.
There are other old varieties of English in Scotland which are more commonly classed as dialects than separate languages, though they can be just as hard to decipher if you're not familar with them.
Doric, nowadays spoken in the North East (Aberdeenshire, etc.), is one example.
To a less extreme extent, the rest of the UK has numerous regional variations of English,
Geordie (spoken on Tyneside in the North East of England) is one well-known example.
By far the majority of people will be diglossic between their regional dialect and
Standard English (and likewise between formal and informal English, using a different register depending on circumstances and who they're conversing with), choosing to use one or the other depending on context.
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