What Does This Band Mean?

Hi all,
Can anyone please tell me what they mean, if someone says "this my phone is two or three bands phone"?

And also, what is the benefit of phone band to the user?

Eeeeeeeeen, what does GSM 900 and GSM 1800 networks or GSM 900/1800/1900 support mean?

most sonyericsson are triband while moto quadband

There is nothing like single band , at least not nowadays
You need two frequency bands! 
People using band combinations such as 850/1800, 900/1900 will experience intermittent signal or no signal at all

.In addition to what infobaba said,
bands could also be defined as frequencies at which the service providers transmit their signals & your handset must be capable to operate at these frequencies in order to function with such
there are also single band phones 1900mhz(work mostly in the US) but most of them wont work here in Nigeria except a few changes if made to the hardware e.g replacing the 1900mhz gsm dcs module with a dual band module =900/1800mhz & flashing the phone with a compatible software . .

@ koseepaaro

If the phone happens to be a single band, forget it, it can never work here (nigeria)

@ dragnet

Who need not to flash such phones, all you do is take it for unlocking 

There are too many inconsistencies here
The facts are:

In GSM system, there is the uplink frequency and the downlink frequency
GSM systems use a pair of frequencies. Modern systems use a set of frequency from one band for uplink and another set from the corresponding band for downlink. e.g 900/1800

Dual Band
Dual band refers to the capability of GSM network infrastructure and handsets to operate across two frequency bands e.g. 850/1900 or 900/1800 MHz. In Nigeria it is 900/1800MHz

Tri Band
A Tri Band phone will operate on three different frequencies depending on the available network. e.g. 850/1800/1900 MHz. (This will not work in Nigeria)
900/1800/1900 will work in Nigeria

Quad Band
A quad-band mobile phone is operational on any of the four GSM frequencies - e.g. 850/900/1800/1900 MHz. This is indeed the ultimate global phone.
And will work on any GSM network in the world

The Single Band i.e 820/850, 1820/1850 (please note that these frequencies are in the same band) will never work here. The cost of conversion to dual band will be more than the cost of a new phone

Nawa oh
  This is 4 nokia 6070 GSM 900 / GSM 1800 / GSM 1900 well i guess most nokia phones hae at least 2 gsm band's. . . . . ^{***not talking 'bout 3310 oh!. . .***} 3310 even get two band's