Researchers have found that adult personality traits are fairly stable throughout life, in a process that starts in childhood. The traits are:
Openness: characterized by imagination, curiosity and seeking out new experiences vs. cautiousness
Conscientiousness: being organized, deliberate and conforming to rules, and social, group or personal norms vs. carelessness
Extraversion: being outgoing, assertive, easygoing, deriving satisfaction from interaction with others vs. aloofness
Agreeableness: being kind, helpful, easygoing, pleasing others vs. unkindness
Neuroticism: being anxious, nervous and self critical and/or in touch with feelings vs. confident
We all have one core trait and we express other traits to some degree, according to situations, cultural context and the people we are with. The Big 5 are found in all cultures and vary according to age and nationality.
How does this affect your romantic relationship? If you and your partner have similar degree of conscientiousness, this is likely to lead to relationship success.
Openness is another trait that contributes to a good relationship, as it promotes a feeling of greater connectedness and satisfaction, because it is linked to the ability to solve conflict and to better communicate.
Life experiences will alter the prevalence of one trait over time in a dynamic process, and you can actually increase your level of openness by practicing with a number of cognitive training tools.
More information about the Big 5 Personality traits: http://www.ocf.berkeley.edu/~johnlab/bigfive.htm
Free Big 5 personality test: http://personality-testing.info/tests/BIG5.php