Gels

Hydrocolloids are those substances that produce a high viscosity solution at low concentration. Some of then have the characteristic of change to gel due to physical (heating) or chemical (addition of calcium or potasium ions) changes.
In brief, a viscous liquid (hydrocolloid solution) becomes a gel after a physical or chemical change.

For instance, agar, κ-carragenan and gelatine can be dispersed in hot water (>90 Celsius in the case of agar, >80 Celsius for κ-carragenan and >50 Celsius for gelatine). They jellify when the temperature drops (<40 Celsius for agar, <30 Celsius for κ-carragenan y <5 Celsius for gelatine).
Methyl-cellulose, instead , can be dispersed in cold water, jellifies when is heated and becomes liquid again when cool down.
Sodium alginate can be dispersed in cold water producing a viscous solution that becomes a calcium alginate gel when calcium ions are added.
These kinds of substances are named as gels, thickeners, hydrocolloids or stabilizers.