The U.K. has officially left the E.U., but there is still a long way to go. Most citizens of the U.K. and E.U. probably won't notice the difference, but Brexit has passed the point of no return.

As of midnight in Brussels on Jan. 31, the U.K. is no longer a member of the E.U. Now the country enters the 11-month transition period during which a new relationship will be negotiated. For now, British passport holders will still be able to travel and work in the E.U., and E.U. citizens will be able to do the same in the U.K. The U.K. will remain in the E.U. single market during the transition period, staying in its custom union and paying into its budget.

So what has changed? The most important change is that the article 50 process is now finished and non-negotiable; the U.K. can't return to the E.U. The U.K.'s half-century membership in the E.U. is officially ended.

The path to this point has not been a smooth one. The citizens of the U.K. voted to leave the E.U. 52% to 48% in June 2016. It was still months before article 50, the start of the Brexit process, was triggered. The first Brexit date was scheduled for March 29, 2019, but Parliament couldn't reach an agreement in time. The deal was finally reached after a change of Prime Ministers and a meeting between new Prime Minister Boris Johnson and Irish Premier Leo Varadkar.

The biggest sticking point in the Brexit negotiations has always been the border between Ireland and Northern Ireland. Historically, this border has historically been a catalyst for violence between those who supported unification with Ireland and loyalists who supported staying with the U.K. Three decades of bombing and shootings resulted in the deaths of 3,500 people.

"The island of Ireland has enjoyed really unbroken peace for 25 years after violence in which 3,500 people died ... and suddenly in the last three years, we have the risk to all that," said Eamon Phoenix, a historian at Queen's University Belfast.

In the agreement reached between Johnson and Varadkar, Northern Irelannd will effectively remain in the E.U.'s single market while the rest of the U.K. leaves. The people of Ireland are not very happy with the deal, and it's not yet clear how it will play out. For starters, Johnson has promised there will be no border checks, while the E.U. has called the checks an "indispensable" part of the agreement. For now, it appears as though a customs border will be established between Great Britain and Northern Ireland. Some goods moving into Northern Ireland will be subject to checks and tariffs, but these would be refunded if the goods stay in Northern Ireland.

The issues still needing to be decided in the U.K.-E.U. relationship moving forward are surrounding law enforcement and data sharing and security, aviation standards and safety, licensing and regulation of medicines, access to fishing waters, and electricity and gas supplies.

Until the end of the transition period, the U.K. will remain under E.U. rules, but they will no longer have a say in what those rules are. Johnson promises that the transition period won't be extended past the end of the year, a short time-frame according to those involved.

Some darker effects of Brexit already being felt in Great Britain. On the same day as Brexit, "racially aggravating" posters telling immigrants who don't speak English to "return" to "the country you came from" were found in the Winchester Tower in Norwich.

The signs told non-English speakers to leave and "let British people live here... We can return to what was normality before you infected this once great island."