The Grignard reagent is an organometallic compound (RMgX). It is a highly reactive substance because of the negative charge present on the R (representative of alkyl, aryl, etc) group.
During the Grignard reaction, conditions must be anhydrous because the Grignard reagent reacts readily with water and forms alkanes.
Grignard reagent must be kept away from water as it reacts readily with water, forming an alkane and MgBr(OH). For this reason, Grignard reagents are usually stored in dry ethers which keep them from hydrolysis.
The alkyl groups of Grignard reactions are electron-rich because of the positive character of metallic groups. These alkyls behave as nucleophiles when attacking other electrophilic carbons. Similarly, they will act as a base when attacking protic species like water, alcohol, etc.