Strings come in a variety of different forms and some have unique features. The path to string nirvana can be a long and expensive journey.
Strings come in a variety of sizes called gauges. The most common gauge is 45-105 for a four string set. Anything lighter than this is considered light and can be as little as 30-90. Lighter strings can have a lighter sound and can need a higher action and less tension on the truss rod. Less common are heavier strings and 50-110 is usually the heaviest available. Heavy strings are useful if tuning down but many players are quite happy tuning 45's down to D-G-C-F.
Think about it - a G string on a 50-110 is the same as a D string on 30-90.
In approximate order of brightness.
These are usually the brightest strings being a stainless steel round wrap over a round or hex core.
These are a softer Nickel steel round wrap.
They have very low tension and come in a very light gauge, like the tone you get when you down tune a bit. They are a bit pricey. They are very low tension, so if you're used to a heavier guage/higher tension string they will feel very different and will probably demand a lighter touch. Very warm sounding with a hint of growl. Some have found that after a few gigs they'd lost “the tone”. Also, unless you have a rock solid neck be prepared for massive adjustments to the truss rod, due to the low tension. Dont string them up and play a gig the following night.
These start off as round wound strings and are either ground or pressed to have a smoother feel like flats with a brighter sound like rounds. They are great for fretless if you want loads of MWAH but dont want to chew up the fingerboard.
These have a sound more like flats.
These are brighter than the Elites and are closer to a nickel round.
These are sometimes called tapewound. Instead of a round wrap they have a flat wrap (like tape). They are often used on fretless as they are easier on the fretboard than roundwounds. If you want that “mwah” sound Jaco Pastorius made famous, you won´t get it with flats. Flats have less highs, some have less sustain than roundwounds. Motown´s James Jamerson played flats on his (fretted) Precision, so that should give you a good idea of how they sound. Big, round, punchy, fat - vintage vibe. Current players using flats include Pino Palladino, MeShell N´degeocello and Raphael Saadiq. Also, harmonics tend to be less resonant with flats. With flats, there´s almost no string noise, so they are more forgiving to play. Though they are more expensive than rounds flats last for years if you don´t break them, so they are actually a better value than roundwounds.
Some players have never found a single set they liked the sound of. They sound 'dead' to them.. as they LOVE that zingy Steels sound.. that piano clang and loads of sustain. Other players use flats to tame an over bright top end. The current trend of super bright amps and light weight basses (which is a necessity for some of us) can lead to too much top.
They can be very stiff and that can make them hard on the fingers. It's also because you're applying pressure to the whole surface area of the string, unlike half/full round strings where you're only really making contact with the round wrap.
Don't be put off by the “Jazz” on the packaging. These are very popular (and expensive) flats that some have on all their basses (fretted & fretless) cos they love the sound and feel of them. They are lower tension compared to many other brands of flats and some find them easier to play and kinder to the fingers. As they have such a loose feel it may be necessary to tweak the truss rod. The TIs seem to last practically forever - well years anyway. There is enough treble to have to roll down the tone control to get some sounds. The TI flats are very big sounding despite being so skinny and can be played with very light finger pressure which makes them a joy on very fast songs. With a fretless FrankenPrecision with an ebony fingerboard set with a very low action, with the tone pot fully open there's a lovely rich organic sustained “mwahh” available. Some find the TI's brighter than other flats but it probably varies with instrument.
As used by James Jamerson. The sound is big round and warm. Can make them punch through if needed, it's down to how you play.
The sound is very old school thump and has that vintage vibe. For a metal band they're a little lacking in top end. Less fret buzz and as soon as you get used to them (or polish the surface of 'em wih playing) they are easy on your fingers! They produce a lovely R+B/Jazz voice.
Very high tension, to the point of being difficult to play. Old school thumpy sound. Standard length strings only just made it over the nut before the silk wrapping starts when used with through body stringing.
There are a number of more exotic strings available.
These are strings that have been coated in non metallic substances. The most popular are Elixirs that are coated with the same stuff as your gore-tex coats. Some come in funky colours.
These are like flats but the wrap is nylon - usually black.
These have a ball at each end for headless basses. Labella do strings developed with Stienberger and of course Status do a wide range of double ball end strings.
Special tunings can sometimes require special strings.
Most of the big string makers only go up to 6 string sets so if you have more strings than that the choice is somewhat limited.