What is the deal with the plus (+) sign in Google search?

It is included as an indication that you can form Boolean search terms.

”+” search (from their website)

Google ignores common words and characters such as where, the, how, and other digits and letters which slow down your search without improving the results. We’ll indicate if a word has been excluded by displaying details on the results page below the search box.

If a common word is essential to getting the results you want, you can include it by putting a ”+” sign in front of it. (Be sure to include a space before the ”+” sign.)

For example, here’s how to ensure that Google includes the “I” in a search for Star Wars, Episode I:

Star Wars Episode +I

The plusses say “this is not a string [words that must appear all together and in order for the search to be correct]” —strings are delimited by quotes. If no words are delimited by either quotes or commas, Google infers commas. “Santa” will be searched separately from “Claus.”

I’m sorry. Clearly I did a terrible job of phrasing my question. I understand the usage of + and quotes while searching based on if I’m looking for a phrase or just key words. What I don’t understand is why Google itself inserts those plus signs every time I search something now. Based on all your answers though, I’m thinking it was just to make it obvious that the search terms aren’t being searched as a phrase but as separate words. Though I think they failed in making that obvious, because those of us who know how to manipulate search already knew that, and those that don’t know aren’t going to understand the point of that now.

Do you mean in the web address? (don’t click the link, this is just an example, the relevant part being “q=Fluther+answers”) Like this?
http://www.google.com/search?q=Fluther+answers

Or does it do that in the text box where you type your query?

In the address, that does not mean ”+Fluther +answers”.

In the text box, I have not seen that. (Google does present different screens & results based on your preferences, history and whatever their engineers decide.)

I don’t see Google alter my search query either. Unless you mean the URL like @jaytkay mentions. In which case:

That’s because an URL can’t contain spaces, they are automatically converted to ’%20’ to prevent the space from corrupting the URL. It’s not that uncommon to change those messy ’%20’ parts of the string to something more human readable like ’+’, and in most cases it’s done as part of a safety precaution (filtering out certain characters or combinations and replace them with other characters that can safely be entered into a database query that won’t harm the database itself; otherwise malicious users could enter database queries as their search query and do all kinds of bad things).

Everything after the first question mark of Google’s URL is called the Uniform Resource Identifier (or URI for short), and is a way to visibly/openly exchange data between two separate pages (in this case Google’s start and results page). The results page has some code to decode the URI to what you originally entered as a search query. So why not use a safer method? Because this method allows you to easily copy/paste URL’s and send them to your friends.

So in other words, it doesn’t really change your query, it just encodes it so it can be used for transport, and decodes it once the transport is done.