The path in the URI has a significance defined by the particular scheme. Typically, it is used to encode a name in a given name space, or an algorithm for accessing an object. In either case, the encoding may use those characters allowed by the BNF syntax, or hexadecimal encoding of other characters.
Some of the reserved characters have special uses as defined here.
The percent sign ("%", ASCII 25 hex) is used as the escape character in the encoding scheme and is never allowed for anything else.
The slash ("/", ASCII 2F hex) character is reserved for the delimiting of substrings whose relationship is hierarchical. This enables partial forms of the URI. Substrings consisting of single or double dots ("." or "..") are similarly reserved.
The significance of the slash between two segments is that the segment of the path to the left is more significant than the segment of the path to the right. ("Significance" in this case refers solely to closeness to the root of the hierarchical structure and makes no value judgement!)
The hash ("#", ASCII 23 hex) character is reserved as a delimiter to separate the URI of an object from a fragment identifier .
The question mark ("?", ASCII 3F hex) is used to delimit the boundary between the URI of a queryable object, and a set of words used to express a query on that object. When this form is used, the combined URI stands for the object which results from the query being applied to the original object.
Within the query string, the plus sign is reserved as shorthand notation for a space. Therefore, real plus signs must be encoded. This method was used to make query URIs easier to pass in systems which did not allow spaces.
The query string represents some operation applied to the object, but this specification gives no common syntax or semantics for it. In practice the syntax and sematics may depend on the scheme and may even on the base URI.
The astersik ("*", ASCII 2A hex) and exclamation mark ("!" , ASCII 21 hex) are reserved for use as having special signifiance within specific schemes.