Types of reagent

Organic reagents are classified into two broad types, based purely on whether they DONATE or ACCEPT electrons when they react with a substrate. A polyatomic ELECTROPHILE (literally, 'electron-loving') is a species that carries an electron-DEFICIENT atom somewhere within it -- that particular atom is called its electrophilic centre. A polyatomic NUCLEOPHILE (literally, 'nucleus-seeking') instead carries an electron-RICH atom somewhere within it -- that atom is called its nucleophilic centre. For instance, in the electrophile $AlCl_3$, the electrophilic centre is the aluminium atom itself, since it has only six valence electrons around it (an incomplete octet, leaving it genuinely electron-hungry); in the nucleophile $H_2O$, the nucleophilic centre is the oxygen atom, since it carries two full lone pairs of electrons it can potentially donate. When a reaction actually happens between the two, a NUCLEOPHILE attacks the electrophilic centre present in a substrate, bringing about what is called a nucleophilic reaction; conversely, an ELECTROPHILE attacks the nucleophilic centre present in a substrate, bringing about what is called an electrophilic reaction. This general principle is illustrated (using a simple, technically inorganic example, purely to demonstrate the idea cleanly) by ammonia reacting with boron trifluoride: $NH_3 + BF_3 \rightarrow H_3N{-}BF_3$, in which ammonia's nitrogen lone pair (the nucleophilic centre in $NH_3$) attacks boron (the electrophilic centre in $BF_3$, which has only six valence electrons around it) to form the product. Within genuinely POLAR organic reactions specifically, this same electrophile/nucleophile framework is what actually drives the chemistry: ELECTROPHILES ($E^{\oplus}$) are electron-SEEKING species, since they are themselves electron-deficient -- examples include an outright positively-charged ion such as $Br^{\oplus} …