The London stock exchange is actually divided into two parts. The first  part is the main market, for which companies need at least three years  of audited accounts to become members. The second part of the exchange  is the Alternative Investment Market, otherwise known as AIM.
AIM offers a market for those shares that either cannot obtain or do not  want a full listing on the main market (also known as the Stock  Exchange's Official List). AIM is targetted at smaller companies with  fewer costs and formalities in obtaining a listing. This often attracts  younger and fast growing companies.
The Alternative Investment Market  started on the London Stock Exchange in June 1995 and at the end of 1996  it replaced the Unlisted Securities Market.
In the early days,  AIM was viewed as a 'high tech' home for UK stocks, rather like the  NASDAQ in America. However, this was in part due to the massive rise in  technology stocks (TMT) in the late 1990s that became the 'Internet  bubble'. 
AIM now hosts a wide range of small companies in almost  every imaginable area - including a number of companies that are not  even UK companies but wanted a UK listing.
It goes without saying  that in many of the companies listed on AIM, liquidity is lower than  might be the case elsewhere on the LSE. This is in large part due to the  smaller relative size of AIM companies, but also because not all  stockbrokers deal in AIM stocks.
How The London Stock Exchange Information System Works?
The London Stock Exchange is the oldest major financial market in the  world but it is now no longer the largest. In March 2005 there were  over 2500 companies quoted on the London Stock Exchange market  including over 300 from outside the UK. 
To be granted permission to be listed on the stock exchange companies must satisfy certain conditions and then others on an ongoing basis. One of the conditions is that companies must have a  trading record of at least three years before they can obtain a listing. 
There are currently two different trading systems: 
SEAQ is the Stock Exchange Automated Quotation System and is a quote-driven  system. Market makers compete, transacting amongst themselves and  dealing with the public through brokers (also known as dealers). Trading  is done by phone. 
  
SETS  is the Stock Exchange Electronic Trading Service and is an order-driven  system. It allows brokers to post orders and trade automatically through  an electronic order book. This eliminated the market makers role. SETS  is limited to FTSE 100 stocks and some FTSE 250 shares.  
London Stock Exchange Information System - SEAQ: 
SEAQ stands for Stock Exchange Automated Quotation System.
Now there are more then 1500 listed companies whose bid and offer prices are quoted on  SEAQ. SEAQ is essentially a price-information service and not a trading  system. It can be accessed through a number of screen based information  services. Trades are therefore carried out either by phone or online. SEAQ provides:
- prices quoted by market makers
- the number of trades reported (above £1,000 in value)
- the best bid/offer prices with upto three market makers prepared to deal
- an automatic trade execution service
If you need more informations please go to SEAQ   
London Stock Exchange Information System - SETS: 
The basis of SETS was launched by the London Stock Exchange in 1997.  It was designed to cater for FTSE 100 firms and the largest of the FTSE  250 companies. Since then the system has (as ever) undergone significant  change. The main points of the SETS system are:
- London  Stock Exchange SETS deals go through brokers. 
- The brokers enter buy and  sell orders through an electronic order book. 
- Market makers have no  role in the transaction - thus removing an added layer of costs.
There  is no minimum order size. This is very useful to the private investor.  The arrival of PEPs and ISAs (both tax advantaged investment plans which  effectively hold shares in a nominee type account) meant that many  private investors might own a portfolio of shares but never receive  annual company accounts. SETS makes it easier to purchase small holdings  (many investors now own a number of shares in their tax wrapper and  just one share in their own name to ensure that corporate documentation  arrives). 
Orders may be matched against more than one opposite  trade in the order book. The Stock Exchange Electronic Trading System  operates on a T+3 basis which means that the financial aspects must be  completed on the third working day after the trade. 
London Stock Exchange SETS orders are either:
- At best which means the trade is carried out at the best possible overall prices
- Execute  and eliminate is also known as immediate execution.  Any amount of an order that can be completed at a set price is done -  everything else is discarded.
- Fill or kill involves the trade being done at exactly the terms specified in the correct volume - or nothing is done at all.
- Limit  orders remain on the SETS screen. They are set for prices that are 'no  worse than' and may be filled slowly over time. An expiry date (max of  90 days) prevents these orders running for too long.
If you need more informations please go to SETS 
Now let's go and see what requirements a company has to meet, before it can be listed on the London Stock Exchange!