Soldering paste or flux
They are all for one purpose - to clean the surfaces to be soldered.

The paste / cream types tend to leave a visible residue which melts further on heating during the soldering process, also it can be helpful to further add paste by dipping the solder into the paste.

The liquid is applied and it tends to dry, just heat and solder away.

Take care to make sure the fluxes are for the task at hand - fluxes for electrical are less aggresive compared to those for plumbing.

Many solders supplied for electrical are already combined with their flux.