The term SAA characters is used generally to mean characters that do not belong to a particular national language but that are nevertheless used regularly on computers. Examples are: currency characters (such as the characters for pound, cent, won, yen, and so on), the vertical broken bar, the not sign, and the overline.
Specifically, SAA refers to the Systems Application Architecture, a published set of interfaces, conventions, and protocols designed to make the development and use of applications consistent across IBM platforms (zOS, iSeries, personal computers).
Unicode is a standard encoding system for representing text. Unicode includes code values for all the national language characters of all nationalities, as well as other symbols.
In unicode each character is represented in 4 bytes. For example, the unicode for the Cherokee letter A is 13a0. Often a unicode value is written with a "0x" preceding it to indicate that the number is hexadecimal: 0x13a0.
For more information see the Unicode Home Page at http://www.unicode.org.