For the Wikipedia policy, see Wikipedia:Case sensitivity.
Text sometimes exhibits case sensitivity; that is, words can differ in meaning based on differing use of uppercase and lowercase letters. Words with capital letters do not always have the same meaning when written with lowercase letters. For example, Bill is the first name of former U.S. president William Clinton, who could sign a bill (which is a proposed law that was approved by Congress). And a Polish person can use polish to clean something. In food, the Calorie, with a capital C, is commonly used to denote 1000 calories of energy.
In computers, some examples of usually case sensitive data are
Some computer languages are case-sensitive (Java, C++, C#, C, Ruby and XML). Others are case-insensitive (i.e., not case-sensitive), such as most BASICs (an exception being BBC BASIC and QBASIC), SQL, Pascal and HTML. There are also languages, such as Haskell and Prolog, in which the capitalization of an identifier encodes information about its semantics.
It takes more work for a program to ignore case when comparing data, depending on the data being compared. Usually it suffices in text coded in character sets like ASCII or EBCDIC to merely convert the comparand and the data temporarily to one case and then compare. However, it becomes far more challenging in a multi-lingual environment, e.g., using Unicode, since case-conversion rules differ between some languages.
Case-insensitive operations are sometimes said to fold case, from the idea of folding the character code table so that upper- and lower-case letters coincide. The alternative smash case is more likely to be used by someone that considers this behaviour a misfeature or in cases wherein one case is actually permanently converted to the other.