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Electrophoresis
An electric field pulls charged molecules through a gel, and smaller ones move faster. Reading the result is a matter of comparing against known sizes.
Agarose and polyacrylamide
Agarose gels suit larger DNA fragments and are quick to pour. Polyacrylamide gels resolve smaller differences and are the usual choice for proteins, where a detergent gives the molecules a uniform charge so that size, not shape, drives separation.
The ladder is the ruler
A lane of fragments of known size run alongside the samples is the only way to estimate what size a band is. Without it, a band's position means very little.
Why bands smear
Degraded samples, too much material in a lane, salt carried over from extraction, or running the gel too hot all produce smears instead of crisp bands. Loading less is often the fastest fix.