My autosomal DNA

All humans have 22 pairs of autosomes and one pair of sex chromosomes (women have two X chromosomes while men have an X and a Y). Here is a video explaining how we inherit autosomal DNA.
(I have tested with two genetic companies. They are 23andMe and Family Tree DNA)

23andMe

23andMe have a feature called Relative Finder which helps you find cousins. In the words of 23andMe -

Relative Finder connects you with 23andMe members who share DNA and ancestors with you.

It can tell you the degree of relationship with your matches due to the size of the segment(s) you both share and the length of the segment(s).

23andMe also gives you health results (Disease Risks, Carrier Status, Traits & Drug Response). Since I'm only concerned with my ancestry I won't go into it in too much detail. 23andMe can tell you if you are a higher risk of getting certain diseases like Alzheimer's Disease or Parkinson's Disease. Even if you are a carrier of certain diseases it doesn't mean that you'll definitely contract them, it just means that you're at a higher risk.

On 23andMe my Ancestry Painting is 100% European.

Ancestry Painting

Ancestry Painting

Here are my Global Similarity results from 23andMe -

Global Similarity

On my Global Similarity: Advanced View; I'm placed in the middle of the Irish box. This isn't too surprising considering I'm Irish.

Global Similarity: Advanced View

Family Tree DNA

Family Tree DNA have a test called Family Finder which tests your autosomal DNA. It works the same as 23andMe's Relative Finder. One of the features on Family Finder is a tool called Population Finder. Like 23andMe's Ancestry Painting it looks at your genetic history and compares your genetics to global populations. Here are my Population Finder results -

Population Finder

The population that I am closest to is Orcadian. It is named after people from the Orkney Islands. This is a term used best to describe people from the British Isles since the populations Family Tree DNA uses for Western Europe are Basque, French, Orcadian and Spanish. It's no surprise that someone who has Irish or British heritage would best match the Orcadians.