Plucked from RedHat Linux 6 Unleashed
read Only= Versus writeable= Versus writable= Versus write ok= (S)
writeable= , writable=, and write ok= are synonyms, meaning they completely substitute for each other. read only= is an inverted synonym for writeable=, writable=, and write ok=, meaning that a read only=yes substitutes for a writeable=no, and a read only=no substitutes for a writeable=yes, and so on. Only one of these four options needs to specify whether a share is writeable. If this option is specified in the [global] section, it serves as a default for all shares. (This is true of all options that can be put in share definitions.) Note that these options can be overridden by the write list= option.
read only=no
writeable=yes
writable=yes
write ok=yes
All four mean the same thing and are interchangeable. The default is read only=yes.