There is no single global standard. The most common systems are:

How it works: Uses separate scales for men, women, and children. A men's size is typically about 1.5 to 2 sizes larger than a women's size for the same foot length (e.g., a Men's US 10 is similar to a Women's US 12).
Peculiarity: The system is based on "barleycorns" (approximately 1/3 of an inch), which is why there's about a one-size difference for every 1/3 inch of length.

How it works: Also uses separate men's and women's scales, but the difference is constant. A UK women's size is 1 size larger than a UK men's size for the same foot length (e.g., a UK Men's 8 is a UK Women's 9).
Important: UK sizes are not the same as US sizes. A US Men's 10 is a UK Men's 9.

How it works: Uses a unisex scale (though fit may be gendered). The size is based on the length of the "last" (the foot-shaped mold a shoe is built on) in centimeters.
The formula is roughly: (Foot length in cm + 1.5) * (2/3) = EU Shoe Size
This system generally has smaller intervals between sizes, allowing for a more precise fit.

How it works: The simplest system. The size number is a direct measurement of the foot length in centimeters (e.g., a 28 cm foot is a size 28). Japan uses the same metric system but with slight variations for width.
This is often considered the most reliable system because it deals with a direct measurement.