This platonic/intimate-yet-unrequited friendship between them, usually called “Chlark” by fans, is one of the fondest, most enduring and complex relationships in the series. 
Clark and Chloe first met in eighth grade when Chloe just moved with her father to Smallville from Metropolis. It was a poorly-hidden secret that Chloe has been in love with Clark in some varying degree since the day they met, but her feelings were largely unrequited as Clark remains emotionally invested in his next-door neighbor Lana Lang (which Pete Ross described as Clark’s “Lana blinders”). However, Clark has admitted that he did have romantic feelings towards Chloe, but was afraid of ruining their friendship if things don’t work out. 
At the end of their freshman year, Clark and Chloe attended the Spring Formal as a couple and almost romantically kissed, but the party was interrupted by a tornado warning, and Clark abruptly abandoned Chloe on the dance floor to rescue Lana. The following day, Clark apologized to Chloe, and she passive-aggressively suggested that they should just remain friends, which (to Chloe’s hidden dismay) Clark actually agreed. The emtional dilemma also proved to be a significant struggle for Chloe, as she also holds dearly her friendship with Lana amongst all her jealousy. Despite all these, Chloe’s feelings for Clark persisted strongly throughout the high school years, and manifested numerous times whenever Chloe was freed from the embarrassment of expressing her love to Clark.
When Chloe started dating in 2006 and later got engaged to Jimmy Olsen in 2008, both she and Clark acknowledged that some things would change between them, but Clark stated that he would accept whatever changes because her happiness was one of the most important things in the world to him. (Plastique)
As one of the first people to learn Clark’s true origins, Chloe has become Clark’s closest friend and ally, and has saved his life multiple times. Clark is also extremely protective of Chloe and has saved her life many times in return, knowing that he can count on Chloe any time he needs her.  However, in Season 9, Clark’s reliance oand trust in Chloe’s instincts shifted drastically after the traumatizing aftermath of his battle with Doomsday. He started rejecting his human emotions and stopped sharing intimate details of his life with her, and they seemed to be more perfunctory than anything else. However, they finally resolved their differences towards the end of the season and gradually rebuilt their friendship.
When Oliver Queen asked Chloe about her feelings for Clark in early 2010, she stated that despite being a very good friend, Clark never managed to fully appreciate her devotion to him, and she had already come to the realization that Clark is not meant for her, to which Oliver commented that not choosing Chloe was “Clark’s loss”. (Escape)
When Desaad attempted to corrupt Chloe by seducing her with a hallucination of Clark, she realized that her romantic feelings for Clark had completely disappeared long ago, and successfully resisted the temptation. (Masquerade)
Although Clark has not always appreciated Chloe as much as she deserves, he has realized over the years that she is an irreplaceable friend and ally, and considers himself lucky to have her in his life. Clark and Chloe have one thing in common, they both know what it felt like to be an outsider.
As young teens, Chloe often wondered out loud how Clark was able to seemingly appear right in the middle of situations or disappear from a room, but she did not openly demand answers, viewing it as part of his introverted personality. Initially, Chloe was assigned to write a school biography about her friend, which manifested in her becoming interested in Clark’s adoption. Clark tolerated the project but when he discovered Chloe was continuing to try to obtain information about him, he reacted with uncharacteristic hostility. Clark demonstrated his powers for Chloe when she was infected with a parasite and she reacted with awe, but forgot them when she was cured. When Lionel Luthor discovered Chloe’s investigation, he manipulated her weaknesses (her crush on Clark and her journalism career dreams) to continue her research. Chloe later maintained payday loans in SC that she did not tell Lionel anything he didn’t already know, and paid dearly for her initial agreement and subsequent refusal to comply.  Chloe’s curiosity was stifled by her ordeal with Lionel and the knowledge that her continued prying would almost certainly end her friendship with Clark, and for the most part, she backed off. However, when exposed to a gas that could make anyone tell the truth, she directly asked Clark what he was hiding (Clark was immune to this ability, though, and could still lie to Chloe), then went to the Kents, sure that they would tell her. Clark rescued and forgave her for her transgression, and she agreed to respect his privacy.