Views lists stuff. The Field UI module lets you add stuff to nodes. Sometimes you need to adjust stuff when working on nodes, users, and other entities. Rules and Actions give you the option to make some changes through a user interface similar to Views.
Actions are built into Drupal. They do not do much out of the box. The actions system is really just a way to label chunks of code that can be attached to nodes and other entities for events. Actions are often supplied by add-on modules. You can write your own actions in your own add-on modules. The main advantage of actions is the ability to activate them from Rules and Triggers.
Triggers, like action, do very little. A trigger can start an action but has no conditional logic. When you want any more than a really simple action, you use rules instead of triggers.
Triggers is an optional module included in Drupal 7 core. The triggers module is switched on by default in the standard installation but is not needed for anything. Some modules will ask you to switch on triggers but most will ask for rules instead of triggers.
A trigger could be the update of a node. The trigger starts an action and the action runs some code in a module then the code decides if the node is the right node and checks any other conditions.
Rules can contain conditional logic. Using our node example, a rule can run for just one content type and only if certain data is present. You quickly switch from triggers to rules. Learn how to use actions then triggers then step up to rules.
Rules can do the same thing as triggers, they can start actions. Rules can also perform some actions themselves without defining actions. Rules can also contain conditional logic.
As a comparison, actions rarely replace modules because actions cannot perform work. Triggers rarely replace modules because triggers cannot make decisions about data. Rules can do both and do replace some simple modules.
When a module sets up processing, the module usually defines the content type and the actions and the rules about performing the actions. You cannot change anything. If the module is simplified to just supply the actions, you can create your own content types and define the rules for when the actions have to occur.
Rules is more complicated than using a module and Rules is more flexible.
Rules has an interface similar to Views. If you like Views and do not like creating modules, Rules is for you.
Learn to use fields, actions, and rules to create the data needed by Views to find the right nodes.