A treaty is the agreement between two or more parties to solve an issue that affects the parties signing the treaty.
A convention is the set of rules for the parties agreeing to the convention to solve an issue that affects larger part of the world.
A treaty enters in force as soon as the parties involved in the treaty signs this agreement. Since a convention affect larger part of the world, it only comes in force when a minimum number of parties agree to the convention (in technical terms ratifies the convention).
Most of the times a treaty is drafted and executed by the parties involved. A convention is drafted and executed by an international body such as "United nations", international maritime organisation or International labour organisation.
For more details read this blog "Treaty and convention: how these two are different"