The new service will run from Hammersmith's Hammersmith and City Line station to Edgware Road station and then join the existing Circle Line at Edgware Road and make a single loop, terminating at Edgware Road station. It will then reverse and do the full circle anti-clockwise, returning to Hammersmith.
TfL's site explains further "On the Tube map the Circle line is shown as a single line. However it shares tracks with the District, Hammersmith & City (H & C) and Metropolitan lines and any disruption on these lines can cause delays to the Circle.
As Circle line trains currently move around the line without a fixed terminus, any delays are compounded over time, and make it harder to offer a regular or reliable service."
No chance of it being named something else though: "It will still be called the Circle line and shown as a circle on the Tube map but with the additional extended service out to Hammersmith."
Basically, the changes will almost double the frequency of trains between Hammersmith and Edgware Road. This will be a big help for people travelling to the recently opened Wood Lane Tube station going to the Westfield shopping centre. But, the new service means some passengers will need to change at Edgware Road to catch the quickest train to their destination.
Overall, the changes are expected to increase reliability and capacity on the Circle, District, Hammersmith and City and Metropolitan lines. Look forward to seeing whether this is the case in December.