Tim Bowen steps forward to offer his explanation of how this versatile word can be used to form several different phrasal verbs.
The CEO of a large company stepped down from his post, however, he could equally well have stepped aside to make room for someone else. In both examples, the meaning is 'leave one's post', generally with the assumption that this is done voluntarily. Now the management of the company will probably want to step back (stop for a while and consider the position) before appointing someone else. Perhaps a suitable candidate will step forward (offer their services) and step into the gap (fill the gap). In order to boost profits, the new chief executive may decide to step up (increase) production or he or she may ask the government to step in (intervene) and help the company in these difficult economic times.
As you can see, step is quite versatile when it comes to forming phrasal verbs. Apart from the examples given above, it can also be used with out to mean ‘to leave a place for a short time’ (as in ‘I’m sorry, Karen’s just stepped out for a moment’). This tends to be used in American English; the British English equivalent is pop out. Step can also be used with on to mean 'to treat someone badly, especially because they have less power or importance', and with up to mean 'to move forwards to a place where an official event is happening’, as in ‘She stepped up to receive her prize'.
Finally, if you are late for an appointment or you think you are going to miss your flight, you might tell your taxi-driver to step on it. This is not a phrasal verb but a potentially reckless way of telling the driver to drive faster!