In the interests of reducing clutter, most ‘footpath furniture’ symbols come with cryptic clues. Here is your key:

A = <should not appear in finished map, but prefix means 'needs attention' - found in comment fields in .mp files>
B = Bridge - normally a Foootbridge, sometimes just a plank! Can also be a road bridge.
C = Culvert (not interesting until it blocks, although it does indicate water sometimes present)
D = Deadfall - large enough to require evasive action. Sometimes whole trees.
E = Encroaching hedge around the Stile/Gate making it hard to use.
F = <should not appear in finished map, but is used to record hard surfaced track/road etc.>  
G = Gate
H = arable Headland (edge of a field).
I = stIle with no step - basically a climbable hurdle. Doesn't meet standard, but there you go .. or not, if you're unathletic.
J = Hedge (HedJe!). Colour coded like FeNces are, although they trend to be harder to get through / over.
K
L in front (Gates only) means gate was locked when I checked. Not very user friendly! Will not actually fail a route unless it is topped with barbed wire or similar. 
M in front means 'Missing'when there should have been (in my opinion). MT = steep bank with no sTeps.
N = Fence (colour coding should show how big a problem it is - Yellow, Red, or Black) 
O in front (Gates only) means Open, so I didn't check it for working order.
P = Fingerpost. Required where a ROW leaves a  County road, but sometimes used elsewhere.
PP= Permissive Path (where the farmer lets you go, having blocked the ROW, I says cynically)
Q = Quagmire. Typically in a gate.
R = <should not appear in finished map, but prefix means 'needs replaced' .. i.e. rather similar to '-'>
S = Stile 
T = sTeps, up/down steep embankments, usually.
U = Upgrowth (brambles, briars, and suchlike, up to and including 80ft Oak trees) on the Route.
V = triVial Ford (can be stepped over with long legs; or Water not always present)
W = Waymark. Normally disks (Blue for Bridleways, Yellow for Footpaths) but coloured tape, paint, and sundry other peculiar things are sometimes used.
X = ROW runs across arable field.
Y = Why? Things like stiles standing in the middle of fields, Bridges with no water
Z

- in front means the next keyed item is defective in some way, e.g.:

-H = Headland ploughed/cropped to within an inch of the boundary.
-W = faded or missing waymark
-S = falling over stiles, unsafe at any weight.
-G = gate requiring two or more strong men to open
-P = Fingerpost barely legible, or trimmed by hedge cutter.
-X = Cross field path is cropped over.
 
Order is not significant, so SWB-S means you’ve got a Stile, Waymark, Bridge and another (sick) Stile, in no particular order (but normally the bridge is in the middle).

? before an item, means the item was not located to best accuracy (+/- 3.0m) and needs to be resurveyed. Sometimes the location of the object (under trees, in gullies) means that best accuracy is never going to be achieved.