The usermodes controlling what you can do on the server, or how the server will handle you. Some usermodes can bet set by yourself, some have to be set by an IRC Operator and some only can be set by the server. Like Channelmodes, Usermodes are case sensitive.

Usermodes:

Privilegies 
o = Global IRC Operator
O = Local IRC Operator
a = Is a Services Administrator
A = Is a Server Administrator
N = Is a Network Administrator
C = Is a Co Administrator

Functions/behavior:
d = Makes it so you can not receive channel PRIVMSGs (Deaf)
g = Can read & send to GlobOps, and LocOps
h = Available for Help (Help Operator)
i = Invisible (Not shown in /WHO searches)
p = Hide all channels in /whois and /who
q = Only U:lines can kick you (Services Admins/Net Admins only)
r = Identifies the nick as being Registered (settable by services only)
s = Can listen to Server notices
t = Says that you are using a /VHOST
v = Receive infected DCC send rejection notices
w = Can listen to Wallop messages
x = Gives the user Hidden Hostname (security)
z = Marks the client as being on a Secure Connection (SSL)
B = Marks you as being a Bot
G = Filters out all Bad words in your messages with <censored> (Note: The Serverbased Badword System is not used at SolidIRC)
H = Hide IRCop status in /WHO and /WHOIS. (IRC Operators only)
R = Allows you to only receive PRIVMSGs/NOTICEs from registered (+r) users
S = For Services only. (Protects them)
T = Prevents you from receiving CTCPs
V = Marks the client as a WebTV user
W = Lets you see when people do a /WHOIS on you (IRC Operators only)