H-1B Cap: The H-1B cap is the Congressionally-mandated limit on the number of individuals who may be granted H-1B status during each fiscal year. For FY08, the cap is 65,000.
Many employers file H-1B petitions on behalf of F-1 academic students after their post-completion OPT expires. Under the previous regulation (and unchanged by this rule), an employer could not file, and USCIS could not approve, an H-1B petition submitted earlier than six months before the date of actual need for the beneficiary’s services or training.
As a result, the earliest date that an employer can file an H-1B petition for consideration under the next fiscal year cap is April 1, for an October 1 employment start date. If that H-1B petition and the accompanying change of status request are approved, the earliest date that the student may start approved H-1B employment is October 1.
Consequently, prior to effective date of this rule, F-1 students who were the beneficiaries of approved H-1B petitions, but whose periods of authorized stay (including authorized periods of post-completion OPT and their grace period) expired before October 1, had to leave the United States, apply for an H-1B visa at a consular post abroad, and then seek readmission to the United States in H-1B status.
Cap gap extension: Under the new rule, the cap gap extension is a period in which an eligible F-1 student’s status is automatically extended to bridge the gap between the end of F-1 status and start of H-1B status. If the student is in a period of authorized post-completion OPT on or after the date the student becomes eligible for the extension, the student’s post-completion OPT is also automatically extended.
Cap gap OPT: A period of OPT that has been extended beyond the date on the F-1 student’s employment authorization document during an authorized cap gap extension.
Also see
http://www.greencardapply.com/news/news08/news08_0507.htm