Hello. WLAN networks (Wireless LAN) refers to wireless LANS in general. Wi-Fi technology would be a specifies.

Thank you sir. Some papers use WLAN instead of Wi-Fi. can you please elaborate the difference in specific to the network specifications. In WLAN we can have Infrared, WiFi etc.,

A wireless local area network (WLAN) is a wireless computer network that links two or more devices using a wireless distribution method (often spread-spectrum or OFDM radio) within a limited area such as a home, school, computer laboratory, or office building.

On the other hand the WIFI is the name of a popular wireless networking technology that uses radio waves to provide wireless high-speed Internet and network connections. A common misconception is that the term Wi-Fi is short for "wireless fidelity," however this is not the case. Wi-Fi is simply a trademarked phrase that means IEEE 802.11x.

I have a response that differs a bit from Kashif's.  Wi-Fi is not the name of a popular wireless networking technology; rather the trademark of the Wi-Fi alliance.  The Wi-Fi alliance uses 802.11 standards as its basis, but does not endorse all of the 802.11 standards. For example, the Wi-Fi consortium has never had a certification for 802.11 frequency hopping spread spectrum (802.11 FHSS is functionally obsolete, but still in use by some) and only a portion of the 802.11i security standard is endorsed/used by the Wi-Fi alliance.

WLAN (Wireless Local Area Network) can be any wireless network.  Arguably, Bluetooth Smart, that supports IPV6 could be considered a WLAN; however, in typical use a LAN uses Ethernet (802.3) as the physical layer and WLAN uses 802.11 (essentially wireless Ethernet) as the physical medium.  Both 802.11 and 802.3 follow the Open Systems Interconnect model, but 802.11 has a different physical layer and a different MAC (media access control ).  The MAC is part of the Data Link Layer.  For example, Ethernet has no need for the concept of roaming that is supported by the 802.11 MAC.   Higher levels of the stack in a 802.11 WLAN and a 802.3 LAN can be identical. For example, a TCP/IP packet sent from an 802.11 device can route to and 802.3 device without any change in the TCP/IP packet.

Just as a WLAN is generic and can mean any wireless LAN, LAN is generic and can mean any local area network, Token Ring, for example.

However, the Wi-FI alliance defines a wireless local area network" (WLAN) product based on the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 standards .  (LINK: http://www.webopedia.com/TERM/W/Wi_Fi.html) .  Because of this definition and the dominance of 802.11 in the WLAN space, it is common today for WLAN and WI-FI to be used interchangeable.  

To be precise, use the standard to which the device communicates, e.g. 802.11ac, 802.11n, 802.11g, 802.11b, 802.11a.  To indicate the portion of 802.11 that the Wi-Fi alliance endorses, one might use "Wi-Fi" and to indicate a wireless local area network (irrespective of the protocol and physical layer), use WLAN.

Thank you Prof.Santiago Cristobal Pérez, Mr.Kashif Naseer Qureshi and Mr.Steven Baker for your valuable comments on my quesiton.

WLAN means wireless local area network which means that two computers are connected through Radio waves rather than a guided medium as in the case of a wired LAN. Two standards IEEE 802.11a which used 5 Ghz frequency band developed in USA and IEEE 802.11b which used 2.4 GHz band developed in Europe were incompatible standards ie the access points, routers and other network devices developed for one standard couldn't be used for other standard. To bring in the compatibility, an alliance of the vendors was created and a word Wi-Fi was coined. This means that network hardware components having Wi-Fi certification could be invariably used for all the standards that emerged afterwards..ie IEEE802.11g, IEEE802.11n etc. I presume this should help

It seems an interesting concept that that IEEE 802.11a was developed in the USA while 802.11b was developed in Europe, when the IEEE is an international standards organization.  The EU did have HiperLAN as an alternate to 802.11 and it is true that 802.11a uses a very similar modulation as HiperLAN2.

Wi-Fi certification really has nothing to do with compatabilty of different PHYsical layers (e.g. 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac).  It is true that many products communicate using multiple PHY layers and so an 802.11a/b/g/n radio can communicate with an 802.11a only AP (and vice-versa).  The IEEE standards were written to force backwards compatiblity of some PHYsical layers, such as 802.11g devices that can communicate with 802.11b devices. 

I am hard pressed to understand how this statement from the prior post: "hardware components having Wi-Fi certification could be invariably used for all the standards that emerged afterwards"  This seems to imply that if a device has Wi-Fi certification it is forward-compatible with all future 802.11 physical layers.  This would indicate that an 802.11b device developed in 1999 running in the 2.4 GHz band should work with a standard developed more than 10 years in the future and running in a different frequency band.

Perhaps what was meant was Wi-Fi certification means that the device is backwards compatible.    Please consider an dissenting example:  The Wi-Fi alliance has separate certifications: Wi-FI Certified (tm) a, Wi-Fi Certified b, Wi-Fi-certified g, Wi-Fi certified n, and Wi-Fi certified ac.  (see attached link and open the tab on the left-hadn column for show advanced filters => connectivity)   There exist wi-fi certified 802.11a ONLY radio cards (e.g. Intel PRO/Wireless 5000 LAN CardBus Adapter Model WCB 5000, Wi-Fi certification ID W000865) and also Wi-Fi certified 802.11g ONLY APs (e.g. D-Link 802.11g Wireless Cardbus Adapter/DWL-G650+) cards. and they CAN NOT communicate one with another.  

Returning to the reason for the formation of the Wi-Fi alliance.  The prior post indicates the Wi-Fi alliance was created to bring compatibility between 802.11a and 802.11b.  However, the problem was more basic than that: WECA (fore-runner to the Wi-Fi alliance) was created to provide testing standards for interoperability between various 802.11b devices with other 802.11b devices.  From Wikipedia: "In 1999, pioneers of a new, higher speed variant, endorsed the IEEE 802.11b specification to form the Wireless Ethernet Compatibility Alliance (WECA) and branded the new technology Wi-Fi"  See second attached link.  

http://www.wi-fi.org/product-finder-results?sort_by=default&sort_order=desc&categories=1&subcategories=23&certifications=1

https://en.wikipedia.org/wiki/Wi-Fi_Alliance