Modern language labs in general offer the following

Text, images, audio and video can easily be integrated; teachers can alter materials to fit their requirements
Learners can record their own voice and play back the recordings, interact with each other and the teacher, and store results
Teachers can intervene and control the learners’ computers via the teacher’s console, track of learners’ work, etc.
Self-access for independent learning which includes access to resources outside class

The purpose of a language lab is to involve students to actively participate in language learning exercises and get more practice than otherwise possible in a traditional classroom environment.
Common components in a modern language lab:

Teacher has a computer with appropriate software for conducting language exercises
Teacher and students wear headsets that block outside sounds and disturbances
Students have a media player/recorder for listening to audio and recording speech
Teacher and student positions are connected via LAN (local area network), in some cases also via separate audio cabling
A server computer or a separate storage device is often used to store lesson materials in a digital format

Why use a language lab in language teaching?

In an average foreign-language class, all students combined speak only 23,5% of the class time (DESI 2006, pg. 48-49).

In a 45-minute long class 23,5% equals approximately 10 ½ minutes. As this number is the speaking time of all students put together, the time each student gets to practice speaking becomes less as class size is increased.

How much speaking practice each student gets during a 45-minute foreign-language class, depending on class size:
    10 student class    30 student class
Traditional classroom    ~1 minute    ~21 seconds
Classroom with language lab    ~11 minutes*    ~11 minutes*

With a language lab, all students in the class can speak simultaneously without distracting each other regardless of the class size. Without a language lab, in a class of more than 10 students, each student gets less than one minute of speaking practice.

*Note: actual speaking time per student can be considerably higher depending on what type of exercises teacher conducts.
Sources

Vanderplank, Robert; 2009; Deja vu? A decade of research on language laboratories, television and video in language learning; Cambridge University Press
DESI; 2006; German Institute for International Educational Research Davies, Graham; Bangs, Paul; Frisby, Roger; Walton, Elizabeth. 2005. Setting up effective digital language laboratories and multimedia ICT suites for MFL. CILT, The National Centre for Languages and the Association for Language Learning

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