Capture the Flag (CTF) competitions can be a great way to practice, hone, and learn offensive information security skills. I hope that this will give you an understanding of what CTFs are and how to get started with them.
There are 3 varieties of CTFs that you may come across - Jeopardy, Attack-Defence and Boot2root.
Jeopardy CTFs are common for online competitions where there will be a number of independent challenges. These challenges are usually grouped into the broad categories of Web, Crypto, Binary, Forensics, Pwning, and Misc. Each challenge is worth a certain number of points, with the more difficult challenges being awarded the most points and the easier beginner challenges less points. At the end of the competition the team with the most points wins.
Jeopardy style CTFs are great for competitor or all skill level and experience as there are usually many challenges at varying degrees of difficulty.
Attack-Defence CTFs are reasonably rare and generally run at a single physical location such as a security conference rather then online due to the complexity of their setup (there are exceptions to this though). Attack-Defence CTFs consist of a number of teams with identical networks, each team is required to run a number of vulnerable services within their network. The aim of Attack-Defence CTFs is to protect your own services while attacking your competitors. You must develop patches for your vulnerable services to try and mitigate your competitors attacks while also developing exploits to run against your competitors services. Points are usually awarded for maintaining performance and uptime of your own services as well as demonstrating successful exploitation of your competitors services.
To compete in a Attack-Defence CTF your team usually has to qualify through a number of Jeopardy CTFs to prove that you have the skills to complete in the big leagues.
Boot2root CTFs are different again, they are more of an individual pursuit rather than a competition amongst teams (although you can still track a team/individuals progress through the challenge by the use of Flags). Boot2root CTFs consist of a vulnerable machine that you need to attack. The goal is to achieve root (administrative) access to the operating system on the server by exploiting a chain of vulnerabilities built into the server. Throughout the challenge you may come across Flags once you have successfully exploited a level within the challenge. The Flags are a useful marking to ensure that you are on the right track (if you can find a successful wrong track through the CTF even better...).
Boot2root CTFs are great as you can download the vulnerable machine image, start it up in your preferred hypervisor (Virtual-box or VMware) and practice in your own time. When you get utterly stuck there are always a number of walkthoughs online that can give you a nudge in the right direction.
CTFtime - CTFtime tracks many of the CTF competitions happening around the world. Keep an eye on upcoming online Jeopardy CTFs, register yourself a team and give them a go.
VulnHub - Vulnhub is a repository for many Boot2root CTFs for you to download and play. Most of the entries also link to walkthoughs/writeups for you to follow if you get stuck.