Reactions are actions that you take out of turn (or possibly on your turn) that happen in response to something else. A reaction can never occur without a trigger.

A Ready action is a special type of action that uses your action to prepare for a triggering event, and when that event occurs you use your reaction to perform whatever you had readied.

To step back a moment, you have four "action types" you can make:

• Action
• Bonus Action
• Reaction
• Movement

On each round, you get one of each of these things. On your turn, you can use an action, a bonus action (if some class feature, feat, spell, or some other thing grants you one), and you can move up to your movement speed (if you use the Dash action you also move as your action). You also get a free "interact with an object" (does not include magic item activation/use). Whenever a triggering event for a reaction occurs, you can also use your reaction. Anything that requires a reaction will tell you what the triggering event is, except in the case of a Ready action, where you actually determine the triggering event yourself.

A Ready action uses an action on your turn to, as I mentioned previously, prepare for some triggered event which then uses your reaction to perform. So when you Ready you might say, "I Ready my firebolt cantrip to cast at the first orc that steps through that doorway."

This uses your action for that turn. Then, when the triggering event occurs, you can do as you have described, using your reaction to complete the Ready action, OR you have the option to move up to your movement you instead (irrespective of if you moved or not on your turn).

So, in essence, a Ready action uses both the action and reaction granted for that combat round, and once your Ready action has triggered you cannot act again until your next turn.

One other note for readying spells -- when you Ready a spell you actually cast the spell using your regular turn action, and "hold" it until the triggering event occurs. Note that this usage requires concentration. You can cast the spell as long as you continue concentrating on holding it.

It is important to note also that because you only get one reaction per combat round that if you have taken an Opportunity Attack or cast a spell with a casting time of reaction (such as Shield) then you cannot use your reaction to complete your ready action (but you might still have movement remaining). On the other hand, if you have taken the Ready action and your trigger has occurred and thus you've used your reaction for that round, then you cannot cast a reaction spell or take any Opportunity Attacks (barring any special abilities/feats that might grant additional OA's).

Your rogue can absolutely attack (action), disengage/dash/etc. (bonus action from Rogue Cunning Action) and Uncanny Dodge (reaction) on a single turn, but he can only Uncanny Dodge in reaction to a triggering event (outlined in the rules for Uncanny Dodge).

Yes

Ready action requires (emphasis mine):

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

The general rule is 1 Action per turn, but then specific rules can override that (e.g. Action Surge, Haste, etc.). I believe that Ready is another specific rule that overrides this as well because while you are using your Action to Ready, what you are really doing is using your Reaction to move your Action to another time.

Attack vs. attack (or does action have a meaning here?)

Upper case and lower case a have a meaning in the PHB, with the former being the Action described under Actions in Combat in the PHB.

When Ready asks you to choose your action, you use the actions listed there. The cases described in the PHB are "Use an Object" and "Movement" for the Ready (since you can only do one.)

In the case of this question, the Action that is being taken is Attack.

Attack Action on your turn

If you take the Attack action on your turn, then it comes with all the fun stuff that happen on your turn. This includes Extra Attack (which typically doesn't happen on a Ready whose trigger occurs NOT on your turn.)

In contrast, you can look at another reaction: Opportunity Attack. In this instance, the language does not say to take an action but to simply make a melee weapon attack. The difference is in the use of Action in the phrasing for Ready.

Actions vs attacks

The language in Ready is to choose the action. Actions are a defined term in the rules under Combat->Actions in Combat.

At other times, there is other language used when specifying melee/ranged/melee weapon/ranged weapon attacks. The language in Ready does not say to Ready an attack (lowercase a), it says to ready an action.

There are also several referenced examples of Ready Action and Extra Attack/Multiattack that reference the On your turn as the important qualifier. Had they not intended a Ready attack to be the Attack Action, then the discussion would have ended there and not referenced the turn requirements.

There are also other specific examples of more than one action on a turn that override that general rule: Cunning Action, Haste, Action Surge, etc.

Intent Support from Jeremy Crawford

The action choice intent (emphasis mine) is also provided by Crawford:

The Ready action lets you ready any action you can take, including Attack, but Extra Attack is on your turn.

Not only has he clearly stated that you take an Action, and that Action can be Attack (uppercase A), but that Extra Attack only functions on your turn.