What is the difference between an 'ale' and a 'bitter'?

Ale - made with a top fermenting yeast, ales are described as "hearty, robust, and fruity."

Bitter - a mainstay in English pubs, this golden-brown draft ale is top-fermented, hoppy, dry, and lightly carbonated.

https://www.merriam-webster.com/words-at-play/from-ale-to-zymurgy-words-about-beer

Originally, ale was not hopped but gained any bitterness - and preservative qualities - from other materials such as heather (still available as Heather Ale - very nice too), gale and even wormwood. Hopping came in the 16th C, and for a long time bitter ale was hopped and mild wasn't. See George Borrow's 'Wild Wales' for interesting comments on hopped and unhopped ales (and the Welsh......). Still in print or at your library, and being well out of copyright, probably available online too. In more modern times, the bitterness was increased, and mild started to be hopped too, but more lightly. Bitter is any colour from pale straw (in the form of India Pale Ale or IPA which is really an export bitter) to golden-brown to true brown. (I'm not referring to the chemical mixture served from little pressurised taps and commonly now referred to as 'smooth' or 'creamy'. This stuff is pale golden brown and is further away from real ale than diet Pepsi is from real Coke...) Mild is usually dark brown, but this is often a matter of caramel, although it can result from chocolate malt. (This is a roasted malt, not a milk drink. Darker is black patent malt.)
Ale also includes winter ale, which as its name suggests is only for winter drinking. A good example is Robinson's Old Tom, which can look darker than Guinness but without the deep head. Often priced by the half rather than the pint for good reason. Then there are brown ales and all sorts of other stuff. As dave says, bitter is but one form of ale. Sources: personal experience (bitter experience at times!);
https://www.seriouseats.com/beginners-guide-to-british-beer-styles-porter-stout-what-is-esb-mild-bitter-ipa-history-england

Recipes.html for alternatives to hops and recipes;
https://en.wikipedia.org/wiki/Ale for different types of ale.

In conclusion, All ale is top-fermented. Bottom-fermented stuff is lager. Lager includes American Budweiser, Coors, Michelob and others no better than the British brewed versions of stuff that's decent drinking in its home land; lager also includes Budvar and other Czech and Polish beers, the main part of German beers and some northern French and Belgian and Dutch brews - which are drinkable. Cains and Samuel Smith in the UK now produce Real Lager, which stops me referring to British lager as ALL rubbish. All ale and lager come into the category 'beer' (drinkability not a criterion); lager and ale are different sets of beer; bitter is a type of ale.

With reference to ZB's post - big brewers may describe their keg beers as 'hearty, robust, and fruity'. Real ale drinkers describe these brews as 'thin and watery, with an aftertaste of the cat' and stick to cask ales. (Kegs are aluminium barrels that are pressurised and gas pressure is used to squirt the stuff out of little taps. Casks are often still wooden, and the ale is pumped up by arm power unless the barrel is in the bar, in which case a spigot is hammered into the bung and the ale drawn direct by gravity.)

author asks: I also heard about 'sour beer'. Where does that fit in?

Baloo answers: Sour beer is a relation of lambic beer (that's Lambic - l and I are sometimes hard to tell apart. Lambic is a Belgian speciality with (to most non-Belgians) rather peculiar additives in the form of fruits. It can take even longer to produce than lager. In both, there is a lactic fermentation that gives the distinctive taste. Personally, not my favourites.
https://www.seriouseats.com/guide-to-belgian-beer-styles-what-is-dubbel-quad-saison-wit-lambic-gueuze

Do not confuse with stale beer. Nowadays, stale beer refers to stuff that's gone off, or lingered as a smell. Originally, stale beer was ale that had matured. Fresh beer tasted mild, beer that was kept (in proper conditions) developed the taste that was preferred. Stale meant then that the liquid had cleared (without the use of finings), and only later acquired the current meaning of 'off'. http://www.breweryhistory.com/journal/archive/112/bh-112-031.html

Incidentally, the author of that last article, Martyn Cornell, is the author of a fairly definitive history of beer that would grace Baloo's shelves, if Baloo had enough shelves and wasn't having to stack things. It's in here somewhere....