Generally, Weeklys have the same contract specifications as standard options, except for the time to expiration. They offer the same continuous, two-sided quotes as standard options. However, they are listed on Thursdays and expire the following Friday. They are intended to offer a cost-effective way to trade around events in a specific time frame.
CBOE was the first options exchange to launch trading on weekly options. Initially Weeklys trading was conducted in on four indexes exclusive to CBOE (SPX, OEX, XSP and XEO). In June 2010, the exchange expanded the program to include listings on indexes, exchange-traded funds (ETFs) and single stocks. 
For an up-to-date list of Weeklys listed on CBOE -- which can change on a weekly basis -- see www.cboe.com/micro/weeklys/availableweeklys.aspx
When Are Weeklys Listed and Not Listed?
New series for Weeklys originally were listed on Fridays and expired the following Friday. As of July 1, 2010, all new Weekly option series at CBOE gained one extra day in the week, beginning trading on Thursdays and expiring the following Friday. The additional day was added to expand market participants' ability to roll trades from one Weekly expiration to the next Weekly expiration. 
No new Weeklys are listed that would expire during the expiration week for standard options (the third Friday of each month). In other words, CBOE does not list new Weeklys on the 2nd Thursday of a month and do not trade Weeklys during the following, standard expiration week until new series are listed on Thursday of that week.
Ticker Symbols for Weeklys are Different
Ticker symbols for Weeklys differ from traditional options contract symbols, with letters identifying the Weekly cycle. For example, the symbol for SPX Weeklys for the first week of the month is JXA, the symbol for the second week is JXB, etc.). 
Weeklys Video Report from CBOE
CBOE offers an informative video update on Weeklys options – covering which Weeklys options classes and strike prices have been the most active and why – every Wednesday, 3 p.m. Chicago time. See http://www.cboe.com/cboetv one of your favorites (choose "Recent" tab after clicking in).