A Kavacha is nothing but a Stotra having a peculiar feature.

[Budha Kavacham][1] (found in Brahma Vaivarta Purana) starts as follows:

<br>

> Asya **Budha kavacha stotra maha manthrasya**
> 
> For the chant called Armour for Budha (mercury)
> 
> Kasyapa Rishi
> 
> The sage is Sage Kasyapa
> 
> Anushtup Chanda
> 
> Meter is Anushtup
> 
> Budho Devatha
> 
> God is Budha (mercury)
> 
> Budha Preethyarthe Jape Viniyoga
> 
> This chant is being done to please God Budha.

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As you can see, it says Budha Kavacha Stotra. That means Kavacha is a Stotra.

Similarly, the [Ketu Kavacham][2] (found in Brahmanda Purana) starts as follows:

<br>

>  अस्य **श्रीकेतुकवचस्तोत्**रमंत्रस्य त्र्यंबक ऋषिः I  अनुष्टप् छन्दः I
> केतुर्देवता I कं बीजं I नमः शक्तिः I  केतुरिति कीलकम् I
> केतुप्रीत्यर्थं जपे विनियोगः II 
> 
> Asya Sri Ketu-**kavacha-stotra**-mantrasya tryamabaka rishih,
> anushtup chandaha, ketur devata etc....
<br>

So, again this also shows that Kavacha is actually a Stotra.

The peculiar feature of such Stotras is that they ask various deities to protect the various limbs of the chanter.

In the Budha Kavacha, you will find that the various names of Budha (like Rohini Suto, Khageswara etc. ) are requested to protect different body parts of the aspirant.

<br>

> Granam gandha priya pathu, jihwam vidhyapriyo mama,<br> Kantam pathu
> vidho, puthro bhujow pusthaka bhooshana., 3
> 
> Let my sense of smell be protected he who likes scents,<br> Let my
> toungue be protected by he who likes knowledge,<br> Let my neck be
> protected by the knowledgable one,<br> Let my arms be protected by he
> who is decorated by books.

<br>

Here Gandhapriya, Vidyapriya, Vidhuputra etc. are various names of Budha, which are requested to protect certain limbs of one who is reading the Kavacha.

Similarly is composed the Ketu Kavacham or any other Kavacha-Stotra in general.

They act on us like how a physical armor will act by protecting us.

<br>

> Is it a mantra?

Yes. it is a Mantra too.

As you can see from the given examples.

Budhakavachamaha**mantra**sya and Ketukavachastotra**mantra**sya --- so a Kavacha is a Stotra and at the same time it is a Mantra as well.

Since, a Mantra that has more than 20 syllables is called a Mala Mantra, therefore any Kavacha will be technically a Mala Mantra.

  [1]: http://www.hindupedia.com/en/Budha_Kavacham
  [2]: http://ioustotra.blogspot.com/2009/10/ketu-kavacham.html