Electronic components

The building blocks you'll find in every radio schematic.

Passive

Resistor (R, in ohms): limits current, creates a voltage drop. See Electricity: the exam basics.
Capacitor (C, in farads): stores energy in an electric field, blocks DC and passes AC ever more easily as frequency rises. Its opposition to current, the reactance, is Xc = 1/(2πfC) — it falls as f rises.
Inductor / coil (L, in henries): stores energy in a magnetic field, passes DC and opposes AC more strongly as frequency rises: XL = 2πfL — it grows with f.

Capacitor and inductor thus behave in opposite ways versus frequency: that's what makes filters and tuned circuits possible (Resonant circuits and filters).

Active (semiconductors)

Diode: conducts in one direction only. Uses: rectification (power supplies), detection, protection. Useful variants: Zener diode (voltage reference), LED, varicap (voltage-controlled capacitance).
Transistor (bipolar or FET): controls a large current/voltage from a small signal → that's amplification and switching. The heart of transmit/receive stages.