Buck Converter

Vin ──[switch]──┬──[ L ]──┬── Vout
                │         │
             diode (or    C    Load
            sync FET)     │
GND ────────────┴─────────┴──

In continuous conduction mode:

$V_{out} = D \cdot V_{in}$

Inductor ripple current: $\Delta I_L = \dfrac{(V_{in} - V_{out})\, D}{L\, f_{sw}}$, typically chosen as 20–40 % of the load current. Modules such as the MP1584 and LM2596 make 12 V → 5 V at several amps easy.