Porto is Portugal's second largest city after the capital, Lisbon, and is located in the north of the country. Despite being one of the main cities in Portugal, Porto is very traditional and provides excellent insight into Portuguese culture. The historical buildings such as the Sé Cathedral and Clérigos Tower are magnificent and there are plenty of cobbled laneways to get lost in.
Porto is positioned on the banks of the Douro River directly across from the city of Vila Nova de Gaia (where you'll find many port wine cellars.) The Dom Luís I bridge connects Porto to Gaia and is a famous landmark in Portugal.
From Porto you can take a day trip to the beautiful Douro Valley, which is where port wine comes from ... and you can't leave Portugal without trying port wine. If you're planning a one week Portugal itinerary (or more) I think Porto and the Douro Valley should be on it. (Don't forget Lisbon too of course.)
I've spent most of my time in Porto so have written a lot about the city. I'm a Porto destination expert for Lonely Planet and have reviewed many of the best hotels in Porto for The Telegraph in the UK. You can find information about where to stay and what to do in Porto in my Stumble Guide.