Yoga Sutras, 2.2 and 2.3

2.2:Samadhi bhavana arthah klesa tanujaranarthah cha, They help us minimize obstacles and attain samadhi.
2.3:Avidya asmita raga dvesha abhinevesah klesah, Ignorance, egoism, attachment, aversion, and clinging to bodily life are the five obstacles.

This is important (and exciting) stuff! In 2.2 They refers to the previous sutra-kriya yoga or the practice of tapas, svadyaya and ishwara pranidhana. The practice of this yoga leads to two things, samadhi (meditative absorption) and minimizing the klesas (pronounced clay-shahs). The establishment of consciousness in samadhi leads to kaivalya (liberation, freedom) which is the goal of yoga according to Patanjali.
The klesas, which Swami Satchidananda translates as obstacles, are usually translated as afflictions. This is an important concept in the philosophy of yoga. It is a bit similar to the concept of oringinal sin in Christianity, more on this in the next sutra. This also returns us to the theme that in yoga we are removing the veil hiding the inner Self, not adding anything else to our self. By removing the klesas, or minimizing them, the inner light of the purusha (Supreme Self) shines through unobstructed.

In Sutra 2.3 the condition of humanity is defined. Avidya (ignorance), asmita (egoism), raga (attachment), dvesha (aversion), and abhinivesah (clinging to life),
are the five klesas, which I prefer to define as afflictions. These are the cause of suffering. Avidya is the biggie. A=not and vidya=knowledge. Avidya is therefore the lack of knowledge of what? Of our true nature, that we are not separate units of consciousness but part of a larger Self. This is a confusing concept for most westerners because we think that knowledge is from books or schools and is conceptual only. Well Patanjali is talking about a whole different type of knowledge. This is not an intellectual understanding that I am God but a full on experience of I am God! Western mystics talk of this, Christ in the bible talks of this unity. According to Patanjali every one can have this experience that "I and the Father are One". He further concludes that all suffering comes from not having this experience of unity. To not feel one with God leads to the next klesa which is asmita, translated as egoism above, which I prefer to translate as the sense of "I am ness". This in turn leads to the next three klesas-seeking fulfillment through pleasure (raga), avoiding any type of unpleasant experience (dvesha), and clinging to the body (abhinivesah). Carrera in his commentary on this sutra says, "The klesas are reminiscent of the notion of original sin in Christianity. Adam and Eve were enjoying communion with God and idyllic life in the Garden of Eden. God's only instructions were that the land be cultivated and that Adam should not eat of the fruit of the Tree of Knowledge of Good and Evil, for in the day that you eat from it you will surely die. (Genesis 2.17)" He goes on to say, "The knowledge referred to in the Bible is the same as the ignorance spoken of in Raja Yoga (another name for Patanjali's yoga). They are two ways to refer to the same experience. Adam and Eve gained knowledge of their individuality, but lost-or became ignorant of-knowledge of God".