Rebel Alliance Ground Forces have ranks & insignia, but they are not all detailed...
As linked by Valorum, there are a number of different ranks. Canon material on Wookieepedia only gives us a brief look at Officer ranks (detailed in Star Wars Rogue One Rebel Dossier and Star Wars: Rogue One: The Ultimate Visual Guide), which are indicated through dots on a plaque. As shown in the Wookieepedia article, in descending order:
There is no information on Enlisted ranks in either the Legends or Canon, so over to speculation...
As shown by several screenshots of rebel soldiers in the Battle of Hoth, most enlisted and specialist had 3 by 3 patches on their right arm (These three pictures courtesy of Wookieepedia):
It can be assumed at this point that all the troopers in this trop
A Rebel technician with an orange and red square on his 3x3 insignia:
After re-watching the Battle of Hoth, it appears that a common enlisted rank is one red (bottom row, first cell) and one blue square in the center (saw this rank a number of times, in all occasions it appeared that soldiers with this patch were taking orders from others.) This screenshot shows several different ranks at once (labelled for reference):
From watching this shot, my speculation is that Numbers 1 and 4 are either troopers/corporals, Number 2 is a Non-Commissioned officer (Sergeant/Sergeant Major/Warrant Officer), also known as an NCO, and No 3 is another enlisted rank either slightly above or below Numbers 1 & 4, possibly in a support role (a trooper/corporal, possibly a machine gunner, medic, etc.) This additional shot shows an enlisted rank shouting to press forward:
It can only be speculated at this point that this trooper is another type of NCO due to there being a solid column of blue on his patch (similar to trooper Number 3 in the previous screenshot.) Another possibility is that these patches are Tactical recognition flashes, patches worn on the sleeve to indicate which regiment/battalion the soldier is from (worn by British army soldiers.)
Please note that this later section is speculation and only the top section about Officer ranks are Canon- This later section is all up for debate. :)