Condoms or barrier contraceptives that prevent conception and STDs come in two forms:

Male condoms, commonly made of latex, are composed of a thin sheath that is worn around a penis during sex.
Female condoms, which are essentially a polyurethane sheath with a flexible ring on each side. A closed end is inserted into the vagina while the open ring sits outside the opening of vagina.

How Do They Work?

The basic principle of a condom is to prevent semen (containing sperm) from entering the vagina and fertilising the egg inside the female partner. When unrolled and worn by the male partner, a condom works to collect the ejaculated sperm inside it.

For female condom, a woman must insert it into her vagina before sexual activity is initiated. The closed ring end of the condom covers the cervix while the other end stays on the outside of vagina. Again, the female condom works similarly to the male one by blocking sperm's entry into the womb.

When speaking of condom effectiveness, we are looking at two parameters:

Birth control
Prevention of STDs

How Effective Are Male Condoms?

According to WebMD.com, male condoms can prevent unplanned pregnancies by 82 per cent. With correct usage, this number could even go higher.
The material of condom plays a role in determining its efficacy against spread of STDs. Polyurethane condoms offer protection, but not as well as a latex condom. Natural or skin condoms do not offer any protection against STDs because their pores are large enough to allow small infectious agents to pass through.
Also, condoms fail to protect against certain STDs that can transmit through skin contact in areas not covered by the condom. These infections include genital herpes or HSV, human papilloma virus (HPV).

How Effective Are Female Condoms?

It is said that a female condom could be 75 - 82 per cent effective in preventing conception.

Female condoms do offer protection against STDs but not as well as male condoms. However, that said, male and female condoms should not be used together, as the friction this causes may make both condoms ineffective.

Ways to Increase Condom Protection

Simply using a condom will not guarantee 100 per cent protection.

Here are some tips to ensure maximum protection:

A condom is meant for a single use. It must be discarded after.
The expiration date on condoms must be checked, as condoms tend to dry up and crack.
Condoms should be kept away from heat and light.
The condom packet must be opened carefully to prevent any tear.

Sources:Image courtesy of [scottchan] at FreeDigitalPhotos.net"Are condoms 100% effective?" https://kidshealth.org/teen/expert/birth_control/effective_condoms.html"Condom Effectiveness: Fact Vs. Fiction," HuffingtonPost.com, Cara Santa Maria, November 17, 2011, https://www.huffingtonpost.com/2011/11/16/condom-effectiveness_n_1098668.html?ir=India"Do condoms really work?" Kidshealth.org, https://kidshealth.org/teen/sexual_health/contraception/expert_condoms_work.html"How does the female condom work?" NHS.uk, https://www.nhs.uk/conditions/contraception-guide/pages/how-does-female-condom-work.aspx"How well do condoms work against STDs?" NBC.news.com, June 29, 2005, https://www.nbcnews.com/id/8399212/ns/health-sexual_health/t/how-well-do-condoms-work-against-stds/#.VKgVUocq7ds"Sexual Health, Birth Control, and Condoms," WebMD.com, https://www.webmd.com/sex/birth-control/birth-control-condoms