The Samaritan woman started off with getting water but realized that she's in for something big. She encountered a Jewish prophet (rarities) who proclaimed living water and portable worship. At the same time, she still has some lingering questions regarding the new concepts. Thus, she proclaimed that the Messiah would further explain everything (4:25). Back in the day, a "Messiah" is some very important person of the Jewish people.
Upon hearing that the woman seeks a Messiah for further help, Jesus saw his chance to proclaim his identity. He declared -- "I am he." (4:26) I could not imagine how amazed the Samaritan woman would be once she realized she was talking to the Messiah the entire time.
It's amazing to see how Jesus went from a run-in with a woman to a proclamation that Jesus is Messiah. Jesus first had an intention to visit an area that barely anyone else would even think of visiting and talk with someone that barely anyone else would even think of talking. He linked the idea of drinking water to living water. When answering questions, he went straight to the point on the core themes regarding the question (rather than superficialities). And once Jesus saw the opportunity to present Jesus, he went straight to the cut.
The entire conversation with the Samaritan woman is often seen as a fine example of sharing Jesus (also known as "evangelism") -- and for good reason! In fact, Jesus made such an impact on the Samaritan woman that her story isn't done yet (as we'll see a few verses later).