Update: The New York Times published an article on this so detailed that it seems to supplant all previous information about this informant, at least pending further information: F.B.I. Used Informant to Investigate Russia Ties to Campaign, Not to Spy, as Trump Claims.
The article tells us (with an unusual amount of detail for identifying a source intended to be kept confidential) that the informant is "an American academic who teaches in Britain" who engaged with George Papadopoulos and Carter Page (and had some prior contacts with Michael Flynn) as part of an FBI national security investigation. The informant paid Papadopoulos to write a research paper and met with him and his assistant to press for information related to the investigation. He also met and exchanged emails with Page.
Based on this reporting, there is no evidence that the informant everyone is presently discussing was "embedded" in the campaign. It appears that this informant was an outsider who asked individuals for information, as opposed to someone who became a part of the campaign. There is no evidence the informant discussed here was a member of the campaign staff, an advisor, or otherwise inside the organization in any way other than talking to individuals (individuals the administration has attempted to distance itself from at that).
We do not know if there were other sources not mentioned here. In one interview, Giuliani claimed there were two informants, but he also acknowledged uncertainty as to whether there were any, as the Times quotes him: "I don’t know for sure, nor does the president, if there really was one."
As for the claim that it is "bigger than Watergate," the article notes some distinctions:
No evidence has emerged that the informant acted improperly when the F.B.I. asked for help in gathering information on the former campaign advisers, or that agents veered from the F.B.I.’s investigative guidelines and began a politically motivated inquiry, which would be illegal.
After opening the Russia inquiry about a month later, they took steps, those officials said, to ensure that details of the inquiry were more closely held than even in a typical national security investigation, including the use of the informant to suss out information from the unsuspecting targets. Sending F.B.I. agents to interview them could have created additional risk that the investigation’s existence would seep into view in the final weeks of a heated presidential race.
F.B.I. officials concluded they had the legal authority to open the investigation after receiving information that Mr. Papadopoulos was told that Moscow had compromising information on Mrs. Clinton in the form of “thousands of emails,” months before WikiLeaks released stolen messages from Democratic officials. As part of the operation, code-named Crossfire Hurricane, the F.B.I. also began investigating Mr. Trump’s campaign chairman, Paul Manafort, and his future national security adviser, Michael T. Flynn.
In other words, the FBI believed it had a legal basis for the investigation based on the evidence it had so far, and took steps to avoid making the investigation public to prevent political consequences during the election.
Update 2: The Washington Post has further information: Secret FBI source for Russia investigation met with three Trump advisers during campaign.
This article adds the additional information that the informant also met with campaign co-chairman Sam Clovis and offered to "provide foreign-policy expertise to the Trump effort." According to Clovis' attorney, the two mainly discussed China. Based on this reporting, I don't believe you could characterize this relationship as "embedded" either. The Post's reporters agree:
There is no evidence to suggest someone was planted with the campaign. The source in question engaged in a months-long pattern of seeking out and meeting three different Trump campaign officials.
The Washington Post has an editorial synthesizing the available public information: What do we know about the ‘FBI informant’ Trump keeps talking about? An important takeaway is that we don't know a lot. What we do know:
The source is a U.S. citizen who has provided information over the years to both the FBI and the CIA … and aided the Russia investigation both before and after Mueller’s appointment in May 2017, according to people familiar with his activities.
That also means this isn’t someone we’ve already heard of.
We also know this from the New York Times, though we don't know it's describing the same source:
At least one government informant met with Mr. Trump’s former campaign aides, Carter Page and George Papadopoulos, current and former officials said.
Another report today, from CNN, includes the claim sourced to anonymous US officials that:
"the confidential intelligence source was not planted inside the campaign to provide information to investigators."
But what we don't know:
We don’t know what the informant’s relationship to the Trump campaign was. Was he or she on the staff? An outside adviser? Just someone who came in contact with people on the campaign for some other reason? We have no idea.
But we have no idea whether the informant was “implanted.” This morning, Trump’s lawyer Rudy Giuliani said, “We’re told there were two infiltrations, two embedded people in the campaign.” Pressed on where he learned this, Giuliani was vague. Given how often he has said things that he quickly walked back or turned out to be bogus, there’s no reason to think he has any idea what he’s talking about. But it’s theoretically possible.
We know essentially nothing about what information the informant provided. We don’t know what the informant has passed on to investigators. We don’t know how central this information became to the investigation, or if it revealed any crimes. The information could be the key to the whole case, or it could be of little or no importance.
In other words, there is not enough public information from reliable sources at this time to understand who all the FBI has received information from. But if today's reports prove accurate, we know that the source was not "planted" inside the campaign.
There is also an issue with the definition of "embedded informant" here. If someone voluntarily reports information and becomes a source because they were, as Simpson said, "concerned," that's not the same thing as the FBI directing someone under its control to join the campaign for the purpose of gathering information. If I witness a stabbing on the street and call the emergency services, I'm not "an embedded informant"; I'm a witness notifying law enforcement about something I see. We don't yet know enough about the source's (or sources') relationship with the campaign and the government to know whether anyone was "embedded" or under what circumstances they came to know what they knew.
In short, there is evidence that there is at least one confidential human source who provided information about the investigation. There is not sufficient reliable public evidence to entirely understand who the source or sources are, their connection to the campaign or law enforcement, how the relationship came to be, or what information they provided. There is at least some information indicating the source was not "planted." Given this, there is also not yet any public evidence to conclude the FBI did anything improper with this source or that it is "bigger than Watergate."