Sessions - A session is the period time a particular user is actively engaged with your website. In general, if the number of sessions is going up over time, that is considered a good thing, as it means there is more traffic to your website.
Pageviews - Pageviews is the total number of pages viewed. Repeated views of a single page are counted. When a visitor hits the back button, a new pageview is recorded. When a visitor hits refresh, a new pageview is recorded. Every time a page is opened in the browser, regardless of whether it has been cached, a pageview is recorded. Increasing pageviews are also a reflection of increasing traffic, hence a good thing.
Unique Pageviews - in the Content parts of Google Analytics, you will sometimes see numbers for "Unique Pageviews." Google defines this as "the number of sessions during which the specified page was viewed at least once. A unique pageview is counted for each page URL + page Title combination." Because Sessions are user-specific, Unique Pageviews represent in a way the number of users viewing this page.
Users - Users who have initiated at least one session during the date range. A user is also a unique combination of a browser and a device. If I visit the same website in a different browser, or on a different device, although I am the same person, I am a different user. This is because Google Analytics puts a browser-specific cookie on each device used, and tracks users with that cookie. Increasing users is considered a good thing.
Mobile usage - percent of Sessions that were on a device other than a desktop computer (i.e., a tablet, phone, etc.).
New Sessions - An estimate of the percentage of first-time visits. Generally a good thing if you have many or increasing new users.
Pages / Session - (Average Page Depth) is the average number of pages viewed during a session. Repeated views of the same page are counted. Higher numbers are considered a good thing, but high numbers could also mean a user is mousing around on your website because they are not finding what they look for.
Avg. Session Duration - The average length of a session. Higher numbers can be positive as it is a measure of engagement, but see pages / session.
Bounce Rate - The percentage of single-page sessions in which there was no interaction with the page. See below for details.
More about Bounce Rate
A bounce is a single-page session on your site. In Analytics, a bounce is calculated specifically as a session that triggers only a single request to the Analytics server, such as when a user opens a single page on your site and then exits without triggering any other requests to the Analytics server during that session.
Bounce rate is single-page sessions divided by all sessions, or the percentage of all sessions on your site in which users viewed only a single page and triggered only a single request to the Analytics server.
These single-page sessions have a session duration of 0 seconds since there are no subsequent hits after the first one that would let Analytics calculate the length of the session.
Is a high bounce rate a bad thing? It depends.
If the success of your site depends on users viewing more than one page, then, yes, a high bounce rate is bad. For example, if your home page is the gateway to the rest of your site (e.g., news articles, product pages, your checkout process) and a high percentage of users are viewing only your home page, then you don’t want a high bounce rate.
On the other hand, if you have a single-page site like a blog, or offer other types of content for which single-page sessions are expected, then a high bounce rate is perfectly normal.