This can stand for any string of characters. CARD could return search hits about wildcards, hardcard, or discard. String searches look for exactly the words or phrases you enter, with no leeway for error or spelling mistakes. As long as you get the spelling right, these are the easiest searches to make. Pages must contain both the phrase and the word California, which could be somewhere else on the page.
Books have indexes.
Your word processor has a command called find. It is impossible to index the Web, simply because it's too large and changes too rapidly. So, if you can't find something, you send out an automated software robot that works on your command. Webcrawler is a spider that searches for keywords in all the text in the document. Other spiders are Lycos, Jump Station, Harvest, and Alta Vista.