Create a polynomial ring using the usual mathematical notation.
i1 : R = QQ[x,y,z];

i2 : R
o2 = R
o2 : PolynomialRing

Notice that after assignment to a global variable, Macaulay2 knows the ring's name, and this name is used when printing the ring.The original description of the ring can be recovered with
describe.
i3 : describe R
o3 = QQ[x..z, Degrees => {3:1}, Heft => {1}, MonomialOrder => {MonomialSize => 32}, DegreeRank => 1]
{GRevLex => {3:1} }
{Position => Up }

Use the following subscript notation to obtain 0,1, or any multiple of 1, as elements in the ring.
i4 : 0_R
o4 = 0
o4 : R

i5 : 1_R
o5 = 1
o5 : R

i6 : 11_R
o6 = 11
o6 : R

Obtain the variables (generators) of the ring by subscripting the name of the ring. As always in Macaulay2, indexing starts at 0.
i7 : R_0^10+R_1^3+R_2
10 3
o7 = x + y + z
o7 : R

The number of variables is provided by
numgens.
i8 : numgens R
o8 = 3

i9 : apply(numgens R, i > R_i^i)
2
o9 = {1, y, z }
o9 : List

i10 : sum(numgens R, i > R_i^i)
2
o10 = z + y + 1
o10 : R

(See
apply and
sum.) Use
generators to obtain a list of the variables of the ring.
i11 : gens R
o11 = {x, y, z}
o11 : List

A matrix (with one row) containing the variables of the ring can be obtained using
vars(Ring).
i12 : vars R
o12 =  x y z 
1 3
o12 : Matrix R < R

The
index of a variable:
i13 : index x, index y, index z
o13 = (0, 1, 2)
o13 : Sequence

The coefficient ring can be recovered with
coefficientRing.
i14 : coefficientRing R
o14 = QQ
o14 : Ring

A random homogeneous element can be obtained with
random.
i15 : random(2,R)
9 2 1 1 2 9 3 2
o15 = x + x*y + y + x*z + y*z + z
2 2 2 4 4
o15 : R

A basis of the subspace of ring elements of a given degree can be obtained in matrix form with
basis.
i16 : basis(2,R)
o16 =  x2 xy xz y2 yz z2 
1 6
o16 : Matrix R < R

We may construct polynomial rings over polynomial rings.
When displaying an element of an iterated polynomial ring, parentheses are used to organize the coefficients recursively, which may themselves be polynomials.
i18 : (a+d+1)^2
2 2
o18 = d + (2a + 2)d + a + 2a + 1
o18 : ZZ[a..c][d..f]

Variable names may be words.
i19 : QQ[rho,sigma,tau];

i20 : (rho  sigma)^2
2 2
o20 = rho  2rho*sigma + sigma
o20 : QQ[rho, sigma, tau]

There are various other ways to specify the variables in a polynomial ring. A sequence of variables can be obtained as follows.
In this example, if you had previously assigned either b or k a value that was not a ring generator, then Macaulay2 would complain about this: it would no longer understand what variables you wanted. To get around this, we could either do
i22 : ZZ[symbol b .. symbol k];

or we may obtain the singleletter variables with
vars.
i23 : vars (0..4)
o23 = (a, b, c, d, e)
o23 : Sequence

i24 : ZZ[vars (0..4),vars(26..30),vars 51]
o24 = ZZ[a..e, A..E, Z]
o24 : PolynomialRing

Subscripted variables can be used, provided the base for the subscripted variable has not been used for something else.
i25 : ZZ[t,p_0,p_1,q_0,q_1];

Sequences of subscripted variables can also be used.
i26 : ZZ[p_(0,0) .. p_(2,1),q_0..q_5]
o26 = ZZ[p ..p , q ..q ]
0,0 2,1 0 5
o26 : PolynomialRing

i27 : (p_(0,0)+q_21)^2
2 2
o27 = p + 2p q + q  2p  2q + 1
0,0 0,0 2 2 0,0 2
o27 : ZZ[p ..p , q ..q ]
0,0 2,1 0 5

The subscripts can be much more general, but care is required when using symbols as subscripts, for the symbols may acquire values later that would interfere with your original use of them as symbols. Thus you should protect symbols that will be used in this way.
i28 : protect xx; protect yy; protect zz;

i31 : ZZ[ee_[xx],ee_[yy],ee_[zz]]
o31 = ZZ[ee , ee , ee ]
[xx] [yy] [zz]
o31 : PolynomialRing

Polynomial rings over polynomial rings work:
i32 : R = QQ[a,b][x]
o32 = R
o32 : PolynomialRing

i33 : (a+b+x)^3
3 2 2 2 3 2 2 3
o33 = x + (3a + 3b)x + (3a + 6a*b + 3b )x + a + 3a b + 3a*b + b
o33 : R

Internally, the polynomials in such towers are expressed in terms of a flattened monoid containing all the variables, obtainable with the key FlatMonoid.
i34 : R.FlatMonoid
o34 = monoid[x, a..b, Degrees => {{1}, 2:{0}}, Heft => {2:1}, MonomialOrder => {MonomialSize => 32}, DegreeRank => 2]
{0} {1} {GRevLex => {1} }
{Position => Up }
{GRevLex => {2:1} }
o34 : GeneralOrderedMonoid

Some things to watch out for when using polynomial rings:

Defining a ring twice gives different rings, as far as Macaulay2 is concerned: We use the strict comparison operator === to demonstrate this.
i35 : ZZ[a,b,c] === ZZ[a,b,c]
o35 = false

Thus it is a good idea to assign a new ring to a variable for future reference.