Pitot Heat is only whilst the aircraft is in the air. On the ground, it will burn the pitot probe out. It is used when there is moisture in the air and there is a risk of ice forming in the pitot tube. Normally it always used whilst transiting through cloud cover. If it is not activated and ice forms in the tube, the pilot will receive incorrect information about the aircraft speed. There are normally 3 Pitot tubes fitted to large aircraft. One for Captains instruments, one for FO instruments and a backup.
De-icing is normally performed on commercial jets by using heated bleed air to melt ice on leading edges of the airframe and around the air intake and N1 fan blade section of the engines. Normally on most jets the nacelette or metalic ring around the front of the air intake is heated to stop ice forming on the engine as cold moist air passes through the N1 stage compressor. Normally engine and wing de-icing is performed when ice is visible on the air frame or windshield. These days most commercial jets have sensors that detect when ice is present. Engine and airframe de-icing can be performed on the ground, but aircraft will still need to be manually de-iced (usually with an alcohol spray) prior to departure when wings and fueselage are covered in ice or snow.
*EDIT* Sorry, forgot, when on the ground, De-icing is only possible with engines running normally! (Unless the aircraft uses a spray system such as the one on the Cessna Citation) Usually after the main engine packs are selected and switched over from the APU pack Wing and Engine De-Icing is selected in low temprature, moist air conditions. (I may be wrong on this as I am going from memory of my checklists but I am sure it is selected before taxi)
