the last three letters appear on the firmware code or name is the firmware version and follows a naming convention by Samsung which represents the build date and version of the firmware. so you can tell by looking at the firmware code which is a newer one (click here for Samsung firmware code reference list)

for example, I9100XWKI4 is an older firmware, I9100XWKI8 is somewhat newer than KI4 (8 is greater than 4), while I9100XWKK5 is newer than KI8 (K is greater then I), etc. but what is the meaning of these three letters?

actually, for instance, the last three letters of I9100XWKI8 is K, I and 8, represents the year, the month of the firmware was built and the versioning respectively. here is the break down for these letters for Samsung Galaxy SII I9100


- the first letter represents the year
J = 2010
K = 2011
L = 2012
M = 2013, etc

so XWKI8 is a 2011 firmware while XXLA4 is a 2012 firmware


- the second letter represents the month
A = January
B = February
C = March
D = April
E = May
F = June
G = July
H = August
I = September
J = October
K = November
L = December

so XWKI8 is built on September 2011, while XXLA4 on January 2012


- the third letter represent the version
so XWKI4 is the forth version, while XWKI8 is the eighth version and XXKPA is the tenth version (A=10, B=11, C=12, etc)


as a result:
- XWKI8 is built on 2011 (K) September (I) and is the 8th (8) version
- XXLA4 is built on 2012 (L) January (A) and is the 4th (4) version



recently u can find some ICS beta versions leaked like I9100XXLP2, I9100XXLP6, I9100XXLPB, etc. the second last letter P refers to pre-release, beta, test built, development, and the like. and u will now know that, XXLPB is built on 2012 (L), is a test built or pre-release (P), and is the eleventh (B) version


UPDATE 18 May 2012:
looks like Samsung now use the letter P for all ICS firmware
for example:
I9100XXKP4 (2011 December), I9100XXLPH (2012 February), I9100XXLPQ (2012 March), I9100BULPC (2012 April), I9100XWLPD (2012 May)
N7000XXLPY (2012 May), N7000ZSLPF (2012 May)

no details about what the P refers to for now




References:
Samsung firmware code reference list
Samsung CSC code reference