The definition of distress is:-
Grave and Imminent Danger to a Person, Ship, Aircraft or Other Vehicle Requiring Immediate Assistance.
Distress is announced as MAYDAY this prefix must only be used for distress traffic and, except in a distress situation the word MAYDAY should never be used on the radio even in conversation.
Emergencies that do not fall into the distress category where immediate assistance is not required but where an urgent message needs to be passed concerning safety of a Person, Ship, Aircraft or Other Vehicle, are URGENCY messages which are prefixed with PAN PAN.
Transmission concerning the safety of navigation are prefixed with SECURITE.

Distress Transmission.
There are three separate parts to a distress transmission.
A. The DSC Distress Alert.
B. The Voice Distress Call.
C. The Voice Distress Message.
Parts A, B and C are used if DSC is fitted. Parts B and C if  it is not.

To send a distress alert from the DSC you should:-
1. lift the cover of the red distress button.
2. press the red distress button momentarily.
3. select the nature of distress “if time allows”  i.e. sinking, fire, collision etc.
4. Depress the red distress button for five seconds or until the unit informs you that the distress message has been sent.
The equipment has now automatically sent an electronic data burst on channel 70 giving;
a) your MMSI
b) your position (from GPS or manual entry)
c) time that the distress was sent.
d) the nature of your distress (if selected)
5. the screen will indicate channel 16 – it is automatically selected for voice communication.
6. the VHF DSC equipment will repeat the message every four minutes until a digital acknowledgement is received or the originating station cancels the alert.
7. wait 15 seconds and then give the voice distress call and message.

Distress calls take priority over all other communications, All stations hearing a MAYDAY must cease all transmissions and continue to listen on the frequency for the distress message.

The format of the distress message is as follows:
Mayday (repeated three times) Your vessels name (repeated three times).
Your International call sign and MMSI.
Mayday (once) your vessels name (once).
Your Position (in Lat and Long) or compass bearings if you have no way of working out your position. Or position unknown.
Nature of Distress ( Sinking, Fire, Collision etc).
I require immediate assistance.
Total number of people on board.
Other useful information.

for example:-

Mayday Mayday Mayday
This is fishing vessel LL307 Early Dawn Early Dawn Early Dawn call sign 2CSG2 MMSI 235075333
Mayday fishing Vessel LL307 Early Dawn call sign 2CSG2 MMSI 235075333
My position is Five Zero degrees, Four Six minutes, North: Zero Zero Five degrees, One Seven minutes, West
I am in collision with a submerged object, I am taking on water and sinking.
I require immediate assistance
One person on board wearing a life jacket
I am abandoning ship.

You can download a copy of the MCA MAYDAY Crib Sheet in .pdf format by clicking Here