These are radiation emition standards. All crt monitors emit some ammont of radiation. Some sweedish organizaton decided to make stadards where the monitors had to emit less than a certain level of radiation in order to pass the standard.
TCO 95 was introduced in 1995 (duh), and TCO 99 was introduced in 99 (DUH). A TCO 99 certified monitor emits less radiation than a TCO 95 monitor.
Though even a TCO 95 monitor emits VERY VERY little radiation, so you shouldn't avoid a monitor just because its only TCO 95. But given the choice, get the TCO 99 instead.
Also, there are other radiation standards called MPR and MPR II. These are similar to TCO 95 and TCO 99. Both standards are extremely acceptable for radiation emition.