There is a level maximum of 20 in Guild Wars.  Each level is based on
experience.  Here is the amount of experience needed to level:

Level      Total XP  Reward
Level 01 -     0
Level 02 -  2000    - 5 Attribute Points, 1 Skill Point
Level 03 -  4600    - 5 Attribute Points, 1 Skill Point
Level 04 -  7800    - 5 Attribute Points, 1 Skill Point
Level 05 - 11600    - 5 Attribute Points, 1 Skill Point
Level 06 - 16000    - 5 Attribute Points, 1 Skill Point
Level 07 - 21000    - 5 Attribute Points, 1 Skill Point
Level 08 - 27600    - 5 Attribute Points, 1 Skill Point
Level 09 - 32800    - 5 Attribute Points, 1 Skill Point
Level 10 - 39600    - 5 Attribute Points, 1 Skill Point
Level 11 - 47000    - 10 Attribute Points, 1 Skill Point
Level 12 - 55000    - 10 Attribute Points, 1 Skill Point
Level 13 - 63600    - 10 Attribute Points, 1 Skill Point
Level 14 - 72800    - 10 Attribute Points, 1 Skill Point
Level 15 - 82600    - 10 Attribute Points, 1 Skill Point
Level 16 - 93000    - 15 Attribute Points, 1 Skill Point
Level 17 -104000    - 15 Attribute Points, 1 Skill Point
Level 18 -115600    - 15 Attribute Points, 1 Skill Point
Level 19 -127800    - 15 Attribute Points, 1 Skill Point
Level 20 -140600    - 15 Attribute Points, 1 Skill Point

You still continue to “level” beyond Level 20, but your level will remain at
20.  Also, you will not get attribute points anymore, but you will still get a
skill point.  All levels after level 20 are 20000 experience apart.  The
attribute points gained from above [170], are all but 30 attribute points.
There are two quests which each give 15 attribute points.  This adds up to 200
total attribute points.

Now, a table showing how attribute points work

  Attribute Level | Points Needed | Total Points
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
    1             |    1          |   1
    2             |    2          |   3
    3             |    3          |   6
    4             |    4          |  10
    5             |    5          |  15
    6             |    6          |  21
    7             |    7          |  28
    8             |    9          |  37
    9             |   11          |  48
   10             |   13          |  61
   11             |   16          |  77
   12             |   20          |  97

As you can see, as your attribute gets pumped higher and higher, the amount of
points becomes steeper and steeper.  Because of this, not all attribute can be
maxed.  I would recommend using three attributes on your character.  There are
many way you can divide up the 200 total attribute points should you choose to
do this.  The most common is 11-11-8.  Another common is 11-10-10.  If you
intend to only use two attributes, you have it easy, 12-12 with a few
leftovers.  If you wish to do four or more attributes, your character will be
extremely weak in the long run.  The only real option for this are 9-9-9-9 and
it is a very poor option at that.

Do not fret though, if you do not like how your attribute points are spread
out, you can use your refund points.  You get 1 refund point for every 250
experience you earn.  There is a maximum of 24 refund points at a time, so if
you are experimenting with new ideas, make sure you still have a method to earn
experience to change them back in the even you are not satisfied.

—This has changed, you now get unlimited refunds while in town, but none
while not in town, the above paragraph is for nostalgia—

All of these attributes are used to boost your skills.  To access the skills
menu, press K by default.  You can have eight skills loaded at a time.  They
correspond with numbers 1-8 on the number row [above qwerty].  These skills can
only be changed while in towns.  Eight seems like a constricting number at
times, so you will have to choose wisely.

Most skills require energy to use, though some require adrenaline.  Others
require nothing at all.  Energy is stat which does not increase with level and
can only be boosted with items.

Elementalists, Necromancers, Mesmers and Monks all start with 30 energy and 4
arrows of energy regeneration.  Each arrow represents .33 energy regained per
second.  Rangers start with 25 energy and 3 arrows of energy regeneration.
Warriors with 20 and 2 arrows.  The extra arrows on the non-warrior classes
come from their armor.  So unless you are not wearing armor, your energy
regeneration will always be this.  Maximum energy can be increased or decreased
by certain armors.  I will have a detailed list of this in the professions
section of the guide.

Adrenaline on the other hand, is gained by hitting an enemy.  This is the way
most warrior skills charge up for use.  There are other skills which can
increase or decrease adrenaline gains.