This is a Wilson camera - a sealed chamber with some gas in a supersaturated state. When a charged particle goes through the gas, it ionizes molecules along its path, we can't see those molecules, there are not that many of them.

Because the gas is supersaturated, anything that disturbs its homogeneity causes some gas to condensate around that disturbance. The ionized molecules by the particle is that cause, and what we see is that condensation of gas along the particle path.

So the charged particle with lots of kinetic energy goes through the gas, ionizes "few" gas molecules on its way, and a lot of gas condensates into a fog which we observe.

One can identify the particle type by the track shape, a thin track is for light particle, a thick one for heavy. If we apply a magnetic field, a charged particle will have a bend track. The direction of that curvature gives a charge sign. The radius of the curvature gives the momentum, energy, and mass.

Now days, Wilson cameras are not in use anymore for real science projects. The ionization chamber rule the game, where the gas ionization caused by the particle is detected by sensitive electronics, the gas is an amplifier of the electric signal (ionization).

Besides gas, scientists use bubble cameras, the same principle, only instead of gas a supersaturated liquid is used, and the bubbles are observed.

For space missions, where gas and bubble cameras cannot be used due to weight limitations, the nuclear emulsions are used. It's made of layers of the E&M sensitive material in a jelly state. The particles leave permanent tracks that can be observed years after the experiment. Emulsions are also used in long term underground experiments.

Wilson and Compton got a Nobel prize for the Wilson camera. The latest development of these cameras is a TPC, time projection chamber. To be honest, another guy got a Nobel prize for that, but it's based on the same physics as the Wilson camera. It's a super complex system, with gas flow/temperature/pressure controlled, many many electrical sensors delivering time and amplitude. It's what collects data at the LHC experiment. So much data, that it had to be distributed among thousands of computers just to do a preliminary statistical analysis.