Ionization Enthalpy, Electron Gain Enthalpy and Electron Affinity

physics kiddy

Ionization Enthalpy is defined as the amount of energy required to remove an atom's electron (can be valence or consecutive) ..... this definition doesn't sound so complex. The definition of Electron Gain Enthalpy - it is the amount of energy required to add an electron. But, what is Electron Affinity ? One book says it is the negative of electron gain enthalpy. Then, there is some thermodynamic rule saying negative value of electron gain enthalpy indicates that energy has to be supplied and positive value indicates energy is released. My question is how can electron gain enthalpy have both negative and positive values but ionisation enthalpy doesn't have. Also please explain what the differnece between affinity and gain enthalpy is.
Thanks in advance ....