Proteins are one of three essential nutrients that we eat. Proteins are made of amino acids and a complete protein should have all the nine essential amino acids. Your body needs twenty amino acids to function, nine of which needs to be ingested and the rest are made by your own body.
Almost all foods contain all the twenty amino acids in some quantity but what we need to look out for are the proportions of the amino acids. In general food from animal sources tend to have more complete or whole proteins and foods from plants tend to be incomplete unless paired with other food sources. Red meat such as beef are the best source of a whole protein. Other sources include fish, buck wheat, quinoa, hemp, chia seeds, and spirulina.
For vegetarians it is important to understand the need to eat complete proteins. Foods such as soya beans and quinoa are complete proteins by themselves. However, many sources of proteins do not contain all the essential amino acids and need to be paired with other foods. Nuts and seeds, legumes and grains are sources of proteins that are incomplete. They need to be eaten along with other foods in order to make it complete. Examples of this include pairing legumes with rice, hummus and pita bread. Multiple sources of protein do not need to be eaten at the same time and can be eaten anytime during the day.