Mild ale is presicely that; ale which is less bitter - milder - that a pint of bitter.   It isn't necessarily dark, sweet, low strength or having no hop character but more often than not is many of of these.   Historically mild means fresh beer that hasn't been aged.   This beer would be slightly sweeter than aged beer as the longer chain sugars wouldn't have all fermented out yet and certainly cheaper as it wouldn't have taken up the storage space.

Mild has been in decline as a beer style although the reasons for this are arguable.   It cetainly did have a cloth-cap image of working class man's or old man's drink.   As a youth I remember every lager drinker having a story of the landlord of his old local emptying the slops tays into the mild barrel - something as illegal then as today.

Mild was traditionally popular in the areas of Britain with heavy industries like the West Midlands, Southⁱ Wales and Lancashire.   This lower alcohol beer was for refreshment and rehydration of the hard labourer.   Historically "lower alcohol" could still mean way over 5% ABV - the regular ales of the time were stronger still.  You'll have to search to find a mild on tap outside of a beer festival if you aren't in these areas today.

Mild isn't usually bottled as the low strength means it doesn't keep well.   If it is then the strength is raised and traditionally called brown ale.

Examples are;

Banks's Original - used to be called mild but was rebranded to shake the old man's drink image.   By far the best selling mild in the UKⁱ and the majority of the Wolverhamptonⁱ brewery's output.   A pale brown beer that most out-of-towners would call a bitter.

Moorehouse's Black Cat Mild - The 2000 Champion Beer of Britain from Burnley, Lancs.   A black mild with loads of malty, toasty & chocolatey flavour.

Hobsons Mild - 2008 Champion Beer of Britain from Shropshireⁱ.   A dark mild with loads of a rounded chocolatey malty taste.

Highgate Mild - another dark mild from Walsall in the West Midlands.

Brain Dark - Another mild that's had the name mild dropped.   Yet another dark mild from Cardiff.