Each segment represents 6 seconds in the game world; ten segments make up a one minute round. A round presents an opportunity for each character involved in a combat situation to take an action. Each round’s activity begins with the character with the lowest weapon speed, casting time or action speed and then proceeds, in order, from there. When a character’s turn comes up in the initiative sequence, that character performs his entire round’s worth of actions unless the character opts to hold his actions until the end of the round. (For exceptions, see Attacks of Opportunity and Special Initiative Actions.)
For almost all purposes, there is no relevance to the end of a round or the beginning of a round. A round can be a segment of game time starting with the first character to act and ending with the last, but it usually means a span of time from one round to the same initiative count in the next round. Effects that last a certain number of rounds end just before the same initiative count that they began on.