Visitors to Egypt from the UK require a visa to enter Egypt, which can be obtained on arrival at the airport for around £13 (rep's price) or $15 if you go to the booths with dollar signs on. Reps charge $18 if you're paying this way. You can hand over £15 sterling at the dollar booths & receive change in local currency. Rules are slightly different for the Sinai region. 

If you're not from the UK, use the great tool at the following link to check if you need a visa. it works for all nationalities.

If you intend to stay in Sharm El Sheikh, for 14 days or less, then you do not need a visa unless you intend to travel to Cairo, Luxor or Ras mohammed national park, by boat, coach or taxi. The officials don't often ask to see the visa but it has been known & one risks being turned away if it can't be produced.  Staying in Sinai for 15-28 days means that you must purchase a visa on arrival, even if you're not leaving the Sinai peninsular.

If you are only going to your resort in Taba , Nuweiba, Dahab and not on any excursions, you do not need a visa. At the borders you dont get of the coach, (if travelling with a travel company) they get on and check your passport. 

You DO need a visa to go to Ras Mohammed by boat, & for some dive sites;  they can be purchased after arrival by asking your / a rep to arrange this for you. Normally, it can be done in 24-48 hrs but will be at the rep's rate (£13 as in April '09) plus a handling fee; so if you want to go there DO get visa on arrival - particularly important for all you divers out there, as Ras Mohammed is FAB diving.