Various vehicles can carry troops across the battlefield. The advantages of being able to move a squad rapidly from one area to another under the protection of armour plate is obvious. The danger is that troops will be wiped out aboard the transport if it is fired at before they disembark.
The two key moments for a unit aboard a transport are disembarking and embarking.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): A character riding a Juggernaut (or any other mount for that matter) cannot be transported in a Rhino (or a Land Raider for that matter).
Errata — Chapter Approved: Main Rules Clarifications (v2.0): A transport vehicle may only transport the unit it was bought as an upgrade for. Characters that join a unit with a transport may be carried in it too, as long as they will fit of course. Vehicles that are not bought as an upgrade but which have transport capability (ie, Land Raiders and Falcons) can carry anybody they like (though still only one squad at a time plus a character that joins the squad). This is a deliberately harsh ruling, designed to stop players treating transports as some form of Warhammer 40,000 taxi service. There may be specific exemptions to this rule though, the ability of the Ork Kult of Speed mobs to fall back to any transport, for example.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): A unit that buys a transport need not be inside it at the start of the game but must deploy at the same time.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): Models with jump packs may not be passengers in transport vehicles.