Possible values for \fontseries and \fontshape

Series, any combination of weight and width is [in theory] possible:

weight                    width
Ultra Light       ul      Ultra Condensed     uc    
Extra Light       el      Extra Condensed     ec    
Light             l       Condensed            c      
Semi Light        sl      Semi Condensed      sc    
Medium (normal)   m
Semi Bold         sb      Semi Expanded       sx    
Bold              b       Expanded             x 
Extra Bold        eb      Extra Expanded      ex 
Ultra Bold        ub      Ultra Expanded      ux

Shape:

upright (normal)   n 
italic             it
slanted/oblique    sl 
small caps         sc
upright italic     ui
outline            ol 

It depends to the definition of the font, if there are less or more ... For the current TL 2012 we get the following values for the series:

b,bc,bm,bux,bx,c,cl,d,db,dbuc,eb,ebc,ebx,el,f,g,i,l,lc,lux,lx,m,mb,mc,muc,mx,n,nx,
ny,o,s,sb,sbc,sbx,sl,ub,ubc,ubx,ux,w,x,xl,z

and for the shape:

a,ac,b,bc,bl,bx,cal,crml,ds,fic,fr,fsc,g,gi,gu,i,ic,Iit,in,In,Isc,Isl,it,itsc,iv,jm,Jm,
kfn,l,li,n,na,ns,o,oi,oj,ol,os,out,p,r,ro,rs,s,sc,scit,sco,scsl,scui,sf,si,sk,sl,slsc,
so,sr,st,ta,tabnums,textdenomnums,textnumnums,tt,u,ui,uv,w

See comment below where Heiko lists some fonts which refer to the given values.

The standard manual for such questions are fntguide LaTeX2ε font selection

Here you can find the following hint:

The most common values for the font series are:

m             Medium
b             Bold
bx            Bold extended
sb            Semi-bold
c             Condensed

The most common values for the font shape are:

n              Normal (that is ‘upright’ or ‘roman’)
it             Italic 
sl             Slanted (or ‘oblique’)
sc             Caps and small caps

You can select what you want as long as the font file exist.

The accepted answer by Herbert follows the list from The LaTeX Companion (on page 414 of the second edition), except that it doesn’t misleadingly display normal width as m (never appended to the code for the series). The LaTeX2e Font Selection Guide recommends, “Whenever possible, you should use the series and shape names suggested in The LaTeX Companion, since this will make it easier to combine new fonts with existing fonts.” However, the qualifier matters. Sometimes, that list isn’t sufficient.

The “Font Installation Guide” adds two additional weights, mb (between m and sb, usually for Medium if Semibold and Regular or Book also exist) and db (for Demibold, between sb and b). A few packages, including nfssext-cfr, support them. If a font has only one weight between Regular and Bold, I recommend labeling it sb (as, for example, Computer Modern Demi), but many fonts come in Regular, Medium, Semibold and Bold, and in this case m, mb, sb and b are the most standard names for them.

Another common case is Italic or Slanted Small Caps. Existing fonts and packages have used scit, itsc, scsl, slsc, and si as a shape name for them. At least one coder put the line \let\scit\itsc in a font package! Fortunately, the issue becomes obsolete if you use fontspec, because small caps is a font feature in OpenType, not a separate font.