In this fourth chapter of the Categories Aristotle introduces a second division of being, classifying “things said without combination.” When considered in their most elemental form, if they can be, on Aristotle’s view things will be organized into these 10 types of categories. In J.A. Ackrill’s considered opinion Aristotle arrived at these categorial classifications, “by observing that different types of answer are appropriate to different questions.”  However he discovered this system of classification, he introduces them very briefly by exampling them with 2 or 3 instances.
Each of the things said without combination signify either substance (ousia), or quantity (poson), or quality (posos), or relation (pros ti) or where (pou) or when (pote) or being in a position (keisthai) or having (echein) or doing (poiein) or being affected (paschein). And substance is, to speak generally, for example a man, a horse; and quantity for example, two cubits long, three cubits long; and quality is for example, white, grammatical; and relation is for example, double, half, greater; and where is for example, in the Lyceum, in the market; and when for example is yesterday, a year ago; and being in a position for example, reclining, sitting; and having for example, having shoes on, being armored; and to do for example, cutting, burning; and to be affected for example, to be cut, to be burnt.
In any affirmation though, none of the things mentioned is said by itself; rather in the combination of these with each other an affirmation comes about. Each affirmation seems either to be true or false, but none of the things said without combination is true or false, for example man, white, runs, wins (translation mine, Categories, 1b25-2a10). 
So we have rather straightforwardly, a classification that looks like this:
Substance (ousia) example: horse, man
Quantity (poson) example: two cubits long, three cubits long
Quality (posos) example: white, grammatical
Relation (pros ti) example: double, half, greater
Where (pou) example: in the Lyceum, in the market
When (pote) example: yesterday, a year ago
Being in a position (keisthai) example: reclining, sitting
Having (echein) example: having shoes on, being armoured
Doing (poiein) example: cutting, burning
Being affected (paschein) example: to be cut, to be cut
It should be noted that when Aristotle both earlier and in this chapter spoke of “things said without combination,” he must have intended this to mean the objects to which the words refer. We can be confident of this because he gives as an example of one his categories, which are by definition said without combination, “in the Lyceum” which is two words in Greek. Yet, on the supposition that Aristotle was referring to linguistic terms alone, of course using two words is an example of combination. Thus, Aristotle must be referring to the concrete items for which these terms are merely used as designations.
It is mnemonically suggestive that 8 of the 10 categories begin with the letter p (Greek π). This might be similar to the journalistic admonition to remember the “W questions,” i.e. who, when, why, where, what. If so, this might cynically incline us to believe that these categories were chosen because they were easily accessible to Aristotle.
In the second half of this passage, Aristotle wants to emphasis that while these categorial uses are building blocks, they are not of any use when discussing truth or falsity. We cannot discover the truth value of “horse.” In order to make it truth-evaluable we must say, “horse is white” or “horse is grammatical,” for example.
Some interpretative questions:
Is “in accordance with no combination” κατὰ μηδεμίαν συμπλοκὴν used by Aristotle as a circumlocution to avoid adding another predicate, i.e. things that are without combination are either, etc.?
Why does Aristotle preface his introduction with “to speak generally” ὡς τύπῳ εἰπεῖν about substance (or maybe all the categories, it seems ambigious)? Is this just to say the examples he is about to give are not exhaustive, typical, or that we should refrain from inferring too much from limited examples?
Does Aristotle want us to understand, in the second paragraph, translating more literally, “In no affirmation is each of the things mentioned predicated of itself?” Viz. “Horse is horse” If this is so, what does this mean for how he intended his theory of predication to be employed, since we are excluding this meaning?
 Could be translated ‘being.’
 Aristotle’s Categories and De Interpretatione, Oxford, 1963, pg. 79
 Τῶν κατὰ μηδεμίαν συμπλοκὴν λεγομένων ἕκαστον ἤτοι (25)
οὐσίαν σημαίνει ἢ ποσὸν ἢ ποιὸν ἢ πρός τι ἢ ποὺ ἢ ποτὲ ἢ
κεῖσθαι ἢ ἔχειν ἢ ποιεῖν ἢ πάσχειν. ἔστι δὲ οὐσία μὲν ὡς
τύπῳ εἰπεῖν οἷον ἄνθρωπος, ἵππος· ποσὸν δὲ οἷον δίπηχυ,
τρίπηχυ· ποιὸν δὲ οἷον λευκόν, γραμματικόν· πρός τι δὲ
(2a.) οἷον διπλάσιον, ἥμισυ, μεῖζον· ποὺ δὲ οἷον ἐν Λυκείῳ, ἐν
ἀγορᾷ· ποτὲ δὲ οἷον χθές, πέρυσιν· κεῖσθαι δὲ οἷον ἀνάκειται,
κάθηται· ἔχειν δὲ οἷον ὑποδέδεται, ὥπλισται· ποιεῖν δὲ οἷον
τέμνειν, καίειν· πάσχειν δὲ οἷον τέμνεσθαι, καίεσθαι.
ἕκαστον δὲ τῶν εἰρημένων αὐτὸ μὲν καθ’ αὑτὸ ἐν οὐδεμιᾷ κατα- (5)
φάσει λέγεται, τῇ δὲ πρὸς ἄλληλα τούτων συμπλοκῇ
κατάφασις γίγνεται· ἅπασα γὰρ δοκεῖ κατάφασις ἤτοι
ἀληθὴς ἢ ψευδὴς εἶναι, τῶν δὲ κατὰ μηδεμίαν συμ-
πλοκὴν λεγομένων οὐδὲν οὔτε ἀληθὲς οὔτε ψεῦδός ἐστιν,
οἷον ἄνθρωπος, λευκόν, τρέχει, νικᾷ. (10)