On most scheduled airlines, all the downline (i.e. subsequent) segments of a given reservation will be cancelled as soon as you don't turn up for the first flight.
Say, you book NYC-LON return. Then your reservation will be
Segment 1: NYC->LON
Segment 2: LON->NYC.
As soon as you miss the NYC->LON flight (Segment 1), your LON->NYC flight (Segment 2) will automatically be cancelled and you will most likely lose the entire value of the ticket (except for the airport taxes and fuel surcharges, if applicable) or have your fare recalculated (i.e. you'd end up paying for the one-way LON->NYC, which will be very expensive) in order to take the LON->NYC flight.
In some exceptional circumstances I've seen people allowed to travel on the subsequent flights, if they have a good story about missed departures and alternative arrangements etc. But generally, you can't really take the subsequent flights unless you've taken the first flight.
NB. This generally doesn't apply on the no-frills/budget airlines, whose fare structure is simple enough for this practice not to matter at all.