Checking whether given string is substring of other string

[prakhargupta]

I found this code . Can Anyone please explain .

string str (“There are two needles in this haystack.”);
string str2 (“needle”);

if (str.find(str2) != string::npos) {
//… found.
}

What is npos here ?

[Anubhav-Aron-1353657628083250 (Anubhav Aron)]