A publishing company collects income from the use of copyright in the music, arrangement and lyrics of your songs. Publishers earn their money by taking a percentage of royalty monies collected through collection agencies in different territories and also by promoting material to other artists (for cover versions) and to broadcasting companies for use in TV, soundtracks, trailers and films.
When you sign a contract you 'lend' the copyright of your material to the publisher for the time period specified in the contract and agree on an income split. If publishing versus recording sounds confusing, an easy way to think of it is to remember that a publisher leases your songs, music and/or lyrics and a record company buys your sound recordings and these are two different things. For example if you were to record your own version of 'Like a Virgin' into your walkman, you would own that particular sound recording but you would not own the song. It's owned by Billy Steinberg and Tom Kelly because they wrote it though it's probably still on loan to a publishing company. Madonna doesn't get a look in.

As with a recording contract you can expect a publishing contract to cover:
. advances for each option period, recoupable
. a royalty split, of which at least 70% goes to the writer
. the length of the contract, this can last from one song to a number of years
. a 'Moral Rights' clause, this covers your legal right to claim ownership of your songs as well as your right to object to distortion of   your work. Don't sign this away!
GUESS WHAT! Don't sign a publishing contract without seeking independent advice from a Music Industry lawyer
MCPS - the collection agency responsible for collecting mechanical royalties from record companies for the sale of musical works on CDs or other audio formats, broadcast and film productions.
PRS - represents songwriters, composers and publishers by collecting their broadcast and public performance royalties.
PPL - the UK collection society for performers and record companies.
Musicians Union - Scotland's Musicians Union is Europe's largest representative organisation for musicians. Members are entitled to services ranging from free legal advice to personal financial advice and access to thousands of industry contacts. The Musicians Union offers New Dealers a 50% discount on membership fees, the remainder of which can be paid for by JobcentrePlus.