A valid marriage is a real, natural marriage that can be contracted between two non-Christians or between a Christian and a non-Christian. With the permission of the local ordinary, a Catholic can marry a non-Christian in a church but not during a nuptial Mass. A sacramental marriage is a valid marriage between two baptized Christians that receives the sanctifying graces of the sacrament of matrimony. When two baptized Christians validly marry, their marriage is presumed to be sacramental. Sacramental marriage does not occur between two non-Christians or between a Christian and a non-Christian because both parties must be baptized. Baptism is the gateway to the other sacraments.
There is one other category to keep in mind: a legal marriage. A “legal marriage” is a marriage that is recognized by the civil state. Not all legal marriages are recognized by the Church to be valid and sacramental, but they ordinarily are acknowledged to be legal (i.e., contracted in a manner that satisfies the civil state).
When two baptized Christians marry, they receive sanctifying graces through their marriage. A Christian who is validly married to a non-Christian may receive sanctifying graces by other means (e.g., the Eucharist, confession) but he does not receive them through his marriage. Should the non-Christian spouse choose to be baptized, the marriage automatically becomes sacramental upon the non-Christian’s baptism.