Intrustion Detection (IDS) and Prevention Systems (IPS) are network security software and hardware that monitor network and system activity for unauthorized activity. These systems identify malicious activity, log information about them and attempt to block them and report them. These systems can send alarms, drop malicious packets, reset connection, block traffic from the offending IP address, etc.  This type of system is also referred to as a firewall.

These systems can be active or passive. Active systems can take real-time corrective action in response to an attack. Passive systems only monitor, log and provide alerts.

These systems can also network based or host based. Networked systems protects an entire group of computers that are network where as the host based systems protects a single computer.

There are different types of Intrustion Prevention Systems:
-Network-based Intrustion Prevention System(NIPS): monitors the network for suspicious traffic by analyzing protocol activity.

-Wireless Instrustion Prevention Systems (WIPS): monitor wireless neetwork for suspicious traffic by analyzing wireless networking protocol.

-Neetwork Behavior Analysis (NBA): Examines network to identify threats that generate unusual traffic flows, such as distributed denial of service (DDoS) attacks, certain forms of malware and policy violations.

-Host-based Intrustion prevention system (HIPS): Software on a single computer that monitors that computer for suspicious activity by analyzing events occuring within that host.

You can implement this type of system by hiring someone to do it in-house or obtain it as a service from a provider.

Below is a list of service providers that you can implement this service for you.
 

Vendor Description
AT&T Intrusion Detection & Prevention System

 AT&T provides IPS as a service.  The AT&T Security Operations Center provides 24x7surveillance to help detect and respond to unauthorized access attempts as early as possible to help prevent damage to your website and business
CISCO

 CISCO provides a range of IPS devices to choose from.  These systems can be used to protect small or large businesses.  They come with firewall and VPN features.
Clone Systems

 Clone Systems provides managaed Intrustion Prevention Service.  They monitor your network from their Network Operations Security Center to detect, block and log malicious threats and unauthorized access to your resources.
Colt

 Colt managed IPS service deliver a proactive network security service that uses real-time inline protection.
Dell SecureWorks

 Dell offers IPS as a service whereby they monitor and secure your network for a fee.  It saves you the cost of adding someone to your staff to it.
Enterasys Intrustion Prevention System

 Enterasys IPS offers an extensive range of detection capabilities, host-based and network-based deployment options, a portfolio of IPS appliances.
eScope

 eScope provides managed IPS/IDS service.  They monitor your network in real-time.
IBM Managed Intrustion Prevention and Detection Service

 IBM offers a managed IPS/IDS service which provides comprehensive protection for your network and servers by helping to block threats and unauthorized access from internal and external sources.
Infenion Tech

 IPS Management is provided as an additional service when you take other managed services. The service comprises installation, operation, update and maintenance of dedicated hardware and software
McAfee Host Intrustion Prevention for Desktop

 McAfee Host Intrustion Prevention for Desktops safeguards your business against complex security threats that may otherwise be unintentionally introduced or allowed by desktops and laptops.  This is a software package you install on your computers.
Netsecuris

 Netsecuris provides managed IDS/IPS service with your own security device or one of theirs.  They monitor your system 24x7.
nGuard

 nGuard provides 24x7 managed Intrusion Prevention and Detection Services for alow monthly price.  
Symantec Critical System Protection

 Used to secure physical and virtual data centers.  Achieve complete protection for Vmware vSphere, stop zero-day and targeted attacks.
Trustwave

 Trustwave provides managed Intrustion Detection service whereby they monitor your network utilizing trained staff.