What is the difference between EDTA and EGTA ?

Both EDTA and EGTA are chelating agents. They are aminopolycarboxylic acids that have more or less the same properties.

EDTA

Ethylenediaminetetraacetic acid (EDTA) is a chelating agent consisting of six binding sites. It has the capacity to bind and sequester a variety of metal ions (except for alkali metals) such as Ca²⁺, Mg²⁺ and Fe²⁺. EDTA combines with all cations in a 1:1 ratio regardless of the charge on the cation. In laboratory applications, EDTA can be used as a preservative for biological samples. It scavenges for trace amounts of metal ions and prevents them from catalyzing air oxidation in the samples. EDTA has a higher affinity for Mg²⁺ ions compared to EGTA.

EGTA

Ethylene glycol tetraacetic acid (EGTA) is also a chelating agent. Compared to EDTA, it has a higher affinity for calcium ions but a lower affinity for magnesium ions.  Similar to EDTA, EGTA can be used as a buffer to resemble the pH of a living cell. This property of EGTA permits its usage in Tandem Affinity Purification, which is a protein purification technique. EGTA has a higher boiling point than EDTA.