An air parcel can be thought of as a block, bubble, or chunk of air. An air parcel "bubble" is not something that has definite size; however, it does retain its general shape and characteristics as it sinks or rises within the atmosphere.
Air parcels that are warmer than the surrounding air will rise. Most people would assume that is due to the old saying that "warm air rises." That is true, but it doesn't explain the physics of why it happens. As a gas is heated, it expands. Consequently, it becomes less dense. The cooler, denser air sinks down beneath it and spreads out. This helps push the warmer, less dense air upward. The energy that initially warmed the air parcel in the first place originated from the sun. Even if the air parcel was heated because of ground temperatures, the ground is simply radiating heat that it received from the sun. An exception to the sun as a general rule and blanket answer would be air parcels that became heated from volcanic activity or fire.
As the air parcel rises, it expands and cools. The cooling causes the relative humidity of the parcel to increase. The temperatures will continue to fall until the dew point is reached. At this point, the relative humidity reaches 100%, and the parcel is saturated. Condensation will begin occurring, and a cloud forms.