Some languages, as for instance German and Spanish, distinguish very clearly between the static passive, which describes the result of an action and the dynamic passive, which describes the process itself. What is the difference? Let's have a look at this sentence.
The switch is turned on.
What does that sentence really mean? Is the switch on or is someone turning it on? If there is an agent, it is obvious that we are talking about a process.
The switched is turned on by Maria
is a process, Maria is turning on the switch. If we just have The switch is turned on we can't say if we are talking about a process or the result of a process.
static (or resultative) passive: Describes the result of a process.
dynamic (or eventive) passive: Describes the process itself.
In English there are no autonomous forms for each of these types of passive as it is the case in Spanish or German. If you really want to describe a process you can use the continuous form in English because the continuous form describes something as happening in a certain moment, describes a process.
The switch is being turned on.
In Italian the difference between a static and a dynamic passive is not as clear as in Spanish or German, but exists. Even if rarely used, for reasons we will still talk about, there is a special form for the dynamic passive in Italian, venire + participio passato (Il libro letto <=> The book is being read). For further discussion it is therefore useful that you understand the difference between the dynamic passive and the static passive.
Normally the passive voice is constructed with a form of essere + participio perfetto and can be interpreted as a dynamic passive as well as a static passive. (For those who speak Spanish: stare, in Spanish estar, is NEVER used in Italian to form the passive. Pay attention to the fact that the past participle of both verbs, essere and stare, is stato, but only essere can be used to form the passive voice.)
La macchina è stata riparata.
The car was repaired.
The car was being repaired.
As we see the sentence La macchina è stata riparata can be translated with a static passive and a dynamic passive.
If we add an agent, in other words the one who actually realizes the action described by the verb we get a dynamic passive. The agent is added in Italian with the preposition da.
La macchina è stata riparata da lui.
If we add an agent the sentence should be translated with a continuous form.
a) La macchina è stata riparata. The car was / has been repaired.
b) La macchina è stata riparata da lui. The car was being / has been being repaired by him.
(As we already mentioned the passato prossimo can be translated with the simple past or the present perfect, depending on the context. If we suppose a context where the fact that the car has been repaired is relevant for the moment of speaking, we translate with an present perfect. If we suppose that the passato prossimo is just a substitution for the passato remoto not used anymore in nowadays Italian, we use the simple past, which describes, in this context, a finished action in a finished past.)
We will see later that all romance languages and Italian as well have an alternative to the passive voice, il si passivante or il si impersonale and therefore there is no real need for an autonomous form for the passive voice, although it exists. The construction venire + participio passato is used to describe a process.
La macchina viene riparata da lui in questo momento. The car is being repaired by him right now.
In general the difference between essere + participio passato and venire + participio passato is not very big. Used as a dynamic passive one form can be substituted by the other. Only in the present this substitution is not possible because the è riparato refers to the past. If you want to describe something as happening at the moment of speaking you must use viene riparato if you don't want to use the si impersonale about which we are going to speak later. For details see Il si passivante.
Example for si passivante
The cars are being repaired.
(literally: The cars repairs themselves.)
Si riparano le macchine.
We are going to discuss that structure later in detail, but actually the construction is easy to understand. In combination with a reflexive pronoun the subject is the executor as well as the goal of the action described by the verb.
I wash myself.
I see myself.
In the case of the si passivante this concept is extended to situations where the subject can't be the executor of the action described by the verb. It is quite obvious that the apples can't eat themselves, but if they could, it would be a normal use of a reflexive verb.
Si mangiano le mele.
The apples are eaten.
(litereally: The apples eat themselves.)
In detail the si passivante / si impersonale is a little bit more tricky than that, but basically it is easy to understand.