Bullethole - Yes you need a trimmer of some sort as suggested by the replies to your post.
And for your info as to why and the reasoning - when you fire your cartridge the brass will expandagainst the chamber walls forming a pretty good gas seal and'flow' toward the mouth of the case. Successive firing will continue this until the case becomes too long to reload and chamber properly causing the mouth of the case to enter the throat of the barrel. This will cause higher than normal pressures and could be dangerous.
Some cartridge cases need to be trimmed more often than others. There are alot of variables as mentioned such as chamber pressure, cartridge design etc. You would be well advised to check your brass after sizing to be sure the case length meets the dimensions listed in the reloading manual for your particular cartridge. Most manuals give a maximum length and a 'trim to' length.