0/0 means both alleles have the reference genotype, while ./. means that it's unknown. The PS value "." means unphased (which is also why the genotype is 0/0 instead of 0|0 or something like that.

in that page, click on pdf version of 4.1 spec. A pdf will load (http://samtools.github.io/hts-specs/VCFv4.1.pdf). In pdf, navigate to section 1.4.2 (Genotype fields).

0/0 means diploid sample is genotyped, genotype is unphased and both the alleles are same as those of reference.

./. means diploid sample is missing genotype information (i.e either missing call).

copy/pasted from pdf:

GT : genotype, encoded as allele values separated by either of / or |. The allele values are 0 for the reference allele (what is in the REF field), 1 for the first allele listed in ALT, 2 for the second allele list in ALT and so on. For diploid calls examples could be 0/1, 1 | 0, or 1/2, etc. For haploid calls, e.g. on Y, male non-pseudoautosomal X, or mitochondrion, only one allele value should be given; a triploid call might look like 0/0/1. If a call cannot be made for a sample at a given locus, '.' should be specified for each missing allele in the GT field (for example './.' for a diploid genotype and '.' for haploid genotype). The meanings of the separators are as follows (see the PS field below for more details on incorporating phasing information into the genotypes):

/ : genotype unphased
| : genotype phased