'stringA' == 'stringB'

Evaluates to true if stringA equals stringB.

For example:

Condition="'$(CONFIG)'=='DEBUG'"

Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values.

'stringA' != 'stringB'

Evaluates to true if stringA is not equal to stringB.

For example:

Condition="'$(CONFIG)'!='DEBUG'"

Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values.

<, >, <=, >=

Evaluates the numeric values of the operands. Returns true if the relational evaluation is true. Operands must evaluate to a decimal or hexadecimal number. Hexadecimal numbers must begin with "0x".

Note
In XML, the characters < and > must be escaped. The symbol < is represented as &lt;. The symbol > is represented as &gt;.

Exists('stringA')

Evaluates to true if a file or folder with the name stringA exists.

For example:

Condition="!Exists('$(builtdir)')"

Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values.

!

Evaluates to true if the operand evaluates to false.

And

Evaluates to true if both operands evaluate to true.

Or

Evaluates to true if at least one of the operands evaluates to true.

()

Grouping mechanism that evaluates to true if expressions contained inside evaluate to true.