These filters are flat disks made of tough plastic polymer materials. The thin polymer disks contain millions of microscopic holes called pores. You can buy filters with specific pore sizes for different chemistry jobs.
The available pore sizes usually range from 0.025 to 10 micrometers across. A 0.2 micrometer pore size is the standard choice for liquid sterilization. This specific size is tiny enough to trap almost all known bacteria.
Students often confuse mechanical filtering with using extreme heat to kill germs. Boiling a liquid sample will certainly kill any bacteria hiding inside it. However, extreme boiling heat will also destroy many delicate liquid chemical compounds.
Filtering simply traps the live germs on top of the membrane surface. The clean, unheated liquid freely drips down into your sterile collection flask. You must always choose the right filter material for your specific liquid.
A basic cellulose filter works perfectly for simple and safe water solutions. A harsh organic solvent will quickly melt that exact same cellulose filter. You would need a tough Teflon filter for that strong organic solvent.
Millipore is actually a famous brand name owned by a large company. Chemists now use the word to describe any similar plastic membrane filter.