lequel,à + lequel,de + lequel (RELATIVE ADJECTIVE)

Lequel is usually a pronoun, but it can also be a relative adjective. Relative adjectives are placed in front of nouns to indicate a link between that noun and an antecedent (the same noun previously stated or implied)

The basic French relative adjective, lequel, has to agree in gender and number with the noun it modifies. In addition, it contracts with the prepositions à and de



Clarification:

Q. What's the difference between lequel as a relative adjective and lequel as a relative pronoun?

A. The same as the difference between any adjective and pronoun:

The relative adjective precedes a noun: laquelle somme sera...

The relative pronoun replaces a noun: - Avez-vous la clé ? - Laquelle ?


http://french.about.com/od/grammar/a/lequel_2.htm

• Like its English counterpart, a French relative pronoun links a dependent or relative clause to a main clause. 

since  relative pronouns may replace a subject, direct object, indirect object, or preposition,
the French relative pronouns que, qui, lequel, dont, and où.

qui means "who" and que means "that" or "what."
The choice between qui and que as a relative pronoun has nothing to do with the meaning in English, and everything to do with how the word is used


Que replaces the direct object (person or thing) in the dependent clause.
    J'ai acheté le livre. Ma sœur l'a écrit. > J'ai acheté le livre que ma sœur a écrit.
    I bought the book (that) my sister wrote.


Qui replaces the subject (person or thing) in the dependent clause.
Je cherche l'artiste. Il étudie à Paris. > Je cherche l'artiste qui étudie à Paris.
I'm looking for the artist (who is) studying in Paris.


Qui also replaces an indirect object referring to a person* after a preposition,** including prepositions which are required after a given verb or expression.
Je vois une dame. Je travaille avec cette dame.
Je vois une dame avec qui je travaille

Lequel
Lequel or one of its variations replaces an indirect object referring to a thing*
Le livre dans lequel j'ai écrit mon nom... / The book in which I wrote my name.
La ville à laquelle je songe... / The town about which I'm dreaming.

dont vs duquel
How do you know whether to use dont or duquel? You need dont when the preposition is de by itself. You need duquel when de is part of a prepositional phrase, such as près de, à côté de, en face de, etc.

Dont
Dont replaces any person or thing after de:

    Où est le reçu? J'ai besoin du reçu. > Où est le reçu dont j'ai besoin?
    Where is the receipt (that) I need?

Dont can indicate possession:
    Voici l'homme. J'ai trouvé la valise de cet homme. > Voici l'homme dont j'ai trouvé la valise.
    That's the man whose suitcase I found.

Il a écrit trois livres. Deux de ses livres sont des best-sellers. > Il a écrit trois livres, dont deux sont des best-sellers.
He has written three books, two of which are best sellers.


Où
You probably already know that as an interrogative pronoun, où means "where," and that it often means "where" as a relative pronoun as well:

    La boulangerie où j'ai travaillé est à côté de la banque.
    The bakery where I worked is next to the bank. (The bakery [that] I worked at...)

Où can also be used after prepositions.
    Le pays d'où il vient...
    The country (where) he's from..

quand vs où
But as a relative pronoun, où has an additional meaning - it refers to the moment in time something happened: "when." This can be tricky, as French students tend to want to use the interrogative quand here. You can't, because quand is not a relative pronoun. You must use the relative pronoun où

Le moment où nous sommes arrivés...
The moment (that) we arrived.

https://www.thoughtco.com/french-relative-pronouns-1368937