The query component is indicated by the first ? in a URI. “Query string” might be a synonym (this term is not used in the URI standard).
Some examples for HTTP URIs with query components:
http://example.com/foo?bar http://example.com/foo/foo/foo?bar/bar/bar http://example.com/?bar http://example.com/?@bar._=???/1: http://example.com/?bar1=a&bar2=b
(list of allowed characters in the query component)
The “format” of the query component is up to the URI authors. A common convention (but nothing more than a convention, as far as the URI standard is concerned¹) is to use the query component for key-value pairs, aka. parameters, like in the last example above: bar1=a&bar2=b.
Such parameters could also appear in the other URI components, i.e., the path² and the fragment. As far as the URI standard is concerned, it’s up to you which component and which format to use.
Example URI with parameters in the path, the query, and the fragment:
http://example.com/foo;key1=value1?key2=value2#key3=value3
¹ The URI standard says about the query component:
[…] query components are often used to carry identifying information in the form of “key=value” pairs […]
² The URI standard says about the path component:
[…] the semicolon (“;”) and equals (“=”) reserved characters are often used to delimit parameters and parameter values applicable to that segment. The comma (“,”) reserved character is often used for similar purposes.