RADIUS: who you are, what you're allowed to do, what you did
Every mechanism has a step-by-step animation, the configuration explained line by line and the exam notes. Where a parameter changes the outcome, you get a simulator in which you change it yourself.
TL;DR
RADIUS is the protocol through which an access device (NAS: switch, WLC, ASA, BNG) asks a central server “who is this, what are they allowed to do and how long did they stay”. It runs over UDP 1812 (authentication and authorization, in a single reply) and 1813 (accounting), with a shared secret between the NAS and the server that hides the password and signs the replies. The Access-Accept brings the authorization attributes — privilege level, VLAN, dACL, SGT, IP — and CoA lets the server change the policy in the middle of the session. It's the protocol of network access: 802.1X, MAB, wireless, VPN, subscribers. For device administration, TACACS+ is a better fit. Most incidents come from a wrong secret (Reject with PAP, silence with EAP), from a source IP that doesn't match the NAS defined on the server, and from authorization not being enabled on the NAS.
How to read the notes: red = critical for the exam, green = theory, blue = numbers, yellow = pitfalls, cyan = differences between RFCs, Cisco and platforms. In the animations, the configuration lines used in the current step are highlighted automatically; the ← → arrows change the step when the animation is selected.
The analogy, in short
Think of a hotel chain: the front desk (the NAS) doesn't keep the guest list; it calls the central office (the server) with an agreed code and asks “do we let them in?”. The central office answers yes or no and, in the same call, says which room and which services the guest gets. At check-in and check-out, the front desk informs the central office; if the central office learns something new, it calls back and asks for the guest to be moved or removed.
The model holds up to §9. From §10 on (MD5, Blast-RADIUS, RadSec), it breaks; see the full analogy and its limits in §15.