A gabion is a galvanised steel wire cage say: 4' x 4' x 2' (HWD). They have a hinged lid and you fill them with stones and then wire the lid shut. Sounds stupidly simple, and it is but they have some rather useful properties. Each unit is an easy one man lift, place and fill. Once filled, each one nominally becomes a large single block with great drainage properties. You can wire these things together into long rows. They work very well with water courses because they are easy to fix in place and once filled, won't move. Pouring conc. into formwork is a right old pain in a fast flowing river and it is all too easy to lose the finer particulates before the stuff has gone off (set and cured).
You can finish the exposed surfaces in various ways. You can pour soil on top and grass them, pour a bit of low grade conc and gravel for a solid "path". The cages are not particularly pretty but neither are they particularly ugly.
If a single cell fails then it generally won't cause much surrounding failure and is easy to replace or repair. You can embed fancy anchors inside them if you have a lateral thrust to resist that can't be dealt with by sheer mass.
Sleepers and the like are quite convenient but you must consider drainage otherwise they will rot within a few decades. Block backed brickwork needs a decent brickie to lay them and if they fail it is usually rather bad. I'm no brickie but I've just repaired a broken 3' retaining wall at home and it looks a bit shit. I will be hiring a professional to sort it all out in spring. Here a gabion wall is overkill!
I did say home projects above but these things are used everywhere and that includes some pretty huge retaining structures. If you are not a Civil Engineer and need to build a decent sized retaining structure then I highly recommend that you consider gabions first because you are far more likely to get it right first time.